I am thinking of trying to build a PC for the first time. I used to know of a web site that would let you start with one component and then would only show the other compatible components. This would keep you from making mistakes like buying a case that everything wouldn't fit in or buying a motherboard that wouldn't work with the video card you picked.

So to make a long question short does any one know of any U.S. website like this as I can't remember the one I used to look around at?Well,  regretfully I do not know of such a website.  However I do have advice for you.

Read a lot!!  seriously, start with a motherboard,  then look at all the specifications: CPU socket type, fsb (front side bus) memory type, northbridge chipset, video interface (PCIex16 2.0, AGPx8 etc....)  and everything you can find.  Then for EVERY word or acronyms you don't know or understand.  look it up,  learn everything you can about it.

Example =>  motherboard SUPPORTS 16 Gb 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel

look up  DDR (learn what it is,  different types i.e. DDR DDR2 DDR3  and what the number means (it's data transfers / second)

It will take a while but in the long run you won't regret knowing what you are doing when you are inside your computer or when something goes wrong. 

And honestly,  it is really just matching numbers and interfaces between your motherboard and everything else.    Keep ASKING questions here if you really don't understand something and there are plenty of people who would gladly explain things to you.

good luckHere's some ideas: http://www.tomshardware.com/reviews/How-To,4/Build-Your-Own,16/

And take a look here: http://www.tomshardware.com/reviews/building-pc,511.html

Best of luck, and, if you have any questions, post back and someone will be pleased to HELP!

check out www.intel.com for motherboards and processors that work together.

then go to www.kingston.com to see what ram will be best for the motherboard.

then go to www.newegg.com and find a computer case (two major types of cases: atx & micro atx), hard drive, cd/dvd drive, keyboard, mouse, monitor, graphics card, etc. along with the motherboard, cpu, and memory you found at the previous sites.