I'll try to keep this as short and sweet as possible.  I'm trying to figure out what the best choices for a hard drive and memory upgrade are for my pc.  I want to take advantage of some of the deals offered on eBay, and I want to know what I'm doing before I do.

I have an IBM (Lenovo) ThinkCentre A50 8085 21U.  Here's some info from http://www.memory-up.com
System Information
Chipset: Intel 82865G/PE/P DRAM Controller/Host-Hub Interface
Intel(R) 82865G/PE/P/GV/82848P Processor to I/O Controller - 2570
Motherboard: Intel D865GKD
CPU: Intel(R) Pentium(R) 4 CPU 3.00GHz
and some other things I came across:
Serial ATA 1.0 On-Board
Dual Channel Memory supported (but not required)
Non-ECC Non-Registered memory
Memory Speed: PC2700, PC3200 (though http://www.crucial.com says this: DDR memory with support for DDR PC3200,DDR PC2700,DDR PC4000 speeds. Because DDR memory is backward-compatible, you can safely upgrade your system with any of the guaranteed-compatible DDR speeds listed below, even if your manual calls for PC1600 or PC2100 speeds.)

Here's a link to the user guide from the IBM website:
ftp://ftp.software.ibm.com/pc/pccbbs/thinkcentre_pdf/19r0764.pdf
and one to the models technical specs:
http://www.dealtime.com/xPF-IBM-ThinkCentre-A50-P4-3-0GHz-1MB-800MHz-FSB-256MB-40GB-48xCD-PCI-To

I called IBM's "tech" (notice the quotation marks) line twice - once regarding RAM and once regarding the hard drive and got quoted a bunch of crap from the website which I had already looked at.  I should have known better, having worked in a call center before, but whatever - you live, you learn.  I know there is likely more information that needs to be known, and I'm sure I've included some that's totally irrelevant to these questions (not to mention may have asked some questions that might not really make sense or are possibly irrelevant), but tell me what you need to know - and where/how to find it and I'll dig it up.

So here are my questions regarding hard drives:
What is the maximum size hard drive I can put in my computer?
How many can I add without having to "swap" one out?
It says that mine supports SATA type storage, but knowing that SATA is backwards compatible, and knowing that the 1.5 and 3.0Gb types came later, if I was to add, say, a drive with a 3Gb data transfer rate, would my pc use that speed, or "scale" it back to 1 or 1.5Mb?
With the same type of thinking in mind, does it matter if I BUY a 10,000rpm drive, or a 7,500?
Are there any other things I've missed that I could/should be taking into consideration?

And concerning RAM:
If I buy DDR PC4000 will it utilize this or only function at 3.2 Gb/sec?
Does it support DDR2?  And if so, how high a speed can I buy that will actually be utilized, and would there be an advantage of buying, say,
Is it important to buy RAM that is matched for my motherboard, and if so why or how important?
The memory I have in now has a latency of 2.5.  If I get a stick (or a kit) that has one lower, will the new one(s) be confined to the higher latency, or do they work completely independently?
How do I know what is the minumum latency my computer supports?
Does it matter if I get a low/high chip density stick (or kit)?  What's the difference?
Also, the manual says I need memory running on 2.5V power.  Is that written in stone?  Could I use (one of) the same power ?connector(s)? that is/are provided to run one requiring less or more power or would I need to also get another power supply or an adapter of some sort for one differing from 2.5V?

I'm sure I'm missing some stuff, and may remember SOMETHING later, but if there's anything I am missing that you would like to INFORM me of, please feel free!

After trying to research this on my own, I just realized that, for me - a person who knows almost nothing about computer hardware - to do the research required to be able to figure out the answers to my questions, I would have to do a lot of reading - after I hunted down the information -  so I'm hoping this will prove the easier route.  I don't just want to know what I can put in my computer safely or compatibly, I want to be able to figure out what the best I can put in is.  At the same time, I don't want to waste money buying something that will only work with my machine  because it's backwards compatible, wasting extra money on something better than my pc can even utilize to it's potential, you know what I mean?  Thanks ahead of time to anyone who can give assistance.

YT,
VL

(So much for KEEPING it short and sweet! )The best thing is to go to www.crucial.com and see EXACTLY what kind of RAM is needed. It is not the cheapest, but it is GUARANTEED compatible, prompt shipping and a LIFETIME guarantee. I doubt you will get that on an eBay deal. If you want to get it elsewhere at least you will know EXACTLY all of the characteristics to shop for. Speed is not the only thing. If you mix speeds ALL of the RAM will run at the slowest speed of any individual chip. Faster RAM will not do more than what the motherboard bus will allow.

If you are using SATA, and the motherboard has 1.5 SATA connectors, a SATA 3 drive will only run at the max speed supported. It is backward compatible.

A 10000 SATA is much more expensive and the speed is not noticeable except in specialized settings. There would be no advantage in your computer.

Any size drive is fine in a modern computer like yours. Larger drives create more heat which can shorten the lifespan. Warranties vary between 1, 3 and 5 years. For my money I would go for a Seagate with the 5 year warranty. There is no advantage to more than 2 hard drives. Larger drives can become quite expensive. Probably the best bang for the buck is in the 250 gig range. www.newegg.com will have a HUGE selection and probably the best prices.

How's that for starters?Thanks for replying GX1_Man.  And it was good for starters.  Of course, it's going to prompt more questions, but at least I'm on the path to an answer.

Ok.  I've been to Crucial's site and used the memory advisor, but the info it gave was not what I needed.  Or maybe it was and I just don't know how to use it, I dunno.
Maximum Memory:   4096MB
USB Support:   2.x Compliant
Standard Memory:   128, 256, or 512MB  removable (though I know I can use 1024, as well.)
Slots:   4 (4 banks of 1)
Currently Installed Memory:  256 MB
Available Memory Slots:  3
Number of Banks:  4
Dual Channel Support:   N.A. (I know that it is supported)
CPU Manufacturer:  GenuineIntel
CPU Family:  Intel(R) Pentium(R) 4 CPU 3.00GHz Model 3, Stepping 4
CPU Speed:  2992 MHz
System Manufacturer: IBM
System Model: 808521U
Motherboard Manufacturer: Intel
Motherboard Model: D865GKD

Sure they give me compatible upgrade possibilities they sell, but correct me if I'm wrong, just because something is compatible doesn't mean it's the best choice.  Like you indicated, RAM that has a potential for a 4Gb/s dtr may be compatible, but also may be wasted if it will only function in my PC at 3.2.  This is what I want to avoid.  Did I understand you correctly in that any addition to my memory will only run at the slowest memory in my machine's speed, or is this defined by the SATA connectors?  Does this also apply to latency as well?  Or when you said "slowest chip," were you referring to something else?  How can I find out what the maximum I can use is?  And if a higher dtr than the one that came with my PC currently runs at is supported, then if the memory that's in there now is the determining factor for speed, can I swap that one out to make use of this greater potential instead of being confined by it's lower speed?

Now as for the hard drives, your point on rpm's is taken.  And as for adding even more storage at some later point, well, that was mainly asked just out of curiosity.  However, rpm's and str's being different, is there a maximum str that can be taken advantage of?  What is/are the determining factor(s)?  How do I find out this information regarding my computer?

Oh, and the reason I am more interested in buying from eBay than Crucial or somewhere similar is money.  I'm living off a student loan right now which makes that situation a little tight, to say the least.  Really, I probably shouldn't even be spending the money, but I never was very wise.  For about the same as I would spend on the 1GB DDR4000 that came up as an option on crucial.com (US $154.99), I could find a 120GB hard drive with whatever manufacturer's warranty and a 512x2 kit with a lifetime warranty on eBay - both brand new.  I need the break in order to upgrade both, and I really do want to upgrade both.  I'm tired of the lags when I'm doing 10 things at once (figuratively speaking...well, most times ), and I'm tired of having to decide what I want to keep the least on my hard drive when something comes up I want to download or whatever that is too big.  40GB is waaaay too small for me.

Please help me!Well, just keep in mind that you need to make sure that ALL of the characteristics of the RAM need to be what your machine can use. Otherwise you may get something incompatible and then you will not have saved any money on the deal.

The maximum RAM speed you can use is based on the system bus. If you get something too fast for the motherboard, it will not perform any faster.

Sometimes faster RAM is cheaper than what you actually need and if that works, then great. I had an older machine that needed PC2700. To get that would have cost more than PC3200 which works just fine, but it still only runs at the slower speed.

As to the hard drive, the maximum size available would probably be usable in youre machine as it is so recent a build.