There's a lot to this.  It took three weeks and a few posts here, hours of looking at the Asus forums and over £1000 of money spent.  Some good money after bad there, but mostly well spent.

I'll try to keep this brief because there's a lot to it.  If this helps someone else to diagnose a problem or save themselves cash then it's done it's job.  Maybe it will help an advisor here - I certainly learned a lot.

First the original SYSTEM - I'll give you the bits that count:

CPU - AMD Athlon 4200 X2
Ram - 2x 512 Kingston DDR 400 + 2x Corsair 512 DDR400
Mobo - Gigabyte GA-K8N-Pro SLi
GPU - XFX 7800GT PCI-e
HDD - Western Digital SE16 320 Sata II (x2)
PSU - Seasonic S12 500Watt

The whole thing was cooled with a water system and ran close to silent.  The temps were pretty normal under load - perhaps edging towatrds the high side but well within the max temps for GPU and CPU per the manufacturers webside (topping out at maybe 70c.

The PC was on and I was watching the TV.  The pc started making a noise louder than it should - the water was leaking.  Switched off.  Found the LEAK and checked that no liquid had gone insode the case.  It hadn't.  Phew.  Fixed the leak, refilled, switched on - dead.  Nothing.  No PSU fans.  Nothing.  Unplugged a few things, reseated them (remember that apart from the water leaking, and therefore higher temps, nothing else had happened)

The Mobo had died on me once before and gave me the same result, but I was aware that this could also be a PSU issue so I shorted the main pins as recommended on the Seasonic site and the PSU powered up.  So I went and bought a new mobo.  This time, the LED on the mobo lit to indicate power was FLOWING.  When I switched on the PSU, the led flashed but again, zero occurrence.  So I bought a new PSU.  This time the PSU came on and the whole lot got to the point of POST testing but failed with Memory errors (1 long and 3 short beeps on AMI BIOS) and I also still had no video.

So at this stage the PC config is:

CPU - AMD Athlon 4200 X2
Ram - 2x 512 Kingston DDR 400 + 2x Corsair 512 DDR400
Mobo - Asus A8N32 SLi Deluxe
GPU - XFX 7800GT PCI-e
HDD - Western Digital SE16 320 Sata II (x2)
PSU - Jeantech Arctic 600w modular

I borrowed an old PCI graphics card from work - it was a TNT pro (!) and this seemed to work.  So I started shopping for a new GPU.  For old times sake, I decided just to give the 7800 another try and as if by magic it booted.  It worked.

That's where we got to with this thread here.

I started to play again.  Wasn't confident enough that I should put the lid back on yet, but I started to run some tests, update a few drivers etc and suddenly the hard drive glitches.  Then again.  It's the rough EM 'scrape' noise you get when you turn off a computer and which I'd heard before when a Hitachi drive decided to let go on me once.  Again it glitched.  Third time, the computer BSOD's with an nvata.sys error.

I did a search and that told me it was often driver errors but I knew from my previous problem that this was also a possible HD failure.  So i swapped the drive to my spare WD drive.  Started to reload windows (it needed to be configured as a boot disk as it had been secondary up until then) but again the errors started.  i felt sure it was NOT the hard drives but something else.

Using a handy laptop, I downloaded memtest 86 and memtest 86+ and created a boot disk.  I disconnected the hard drives and ran the tests and this gave me errors in Test #6.  No idea what that means, and there's no clear instruction anywhere what it might indicate, but there they were.  Errors in test #6.

I then read that AMD CPU chips have a memory controller onboard.  So what could the problem be?  The CPU, the Mobo, or the memory?  I spoke to a man who had a stall at a computer fair.  His place was not too far from me so i took him all three and he ran them OVERNIGHT.  he recommended replacing the RAM with some ECC memory.  he took in the old RAM and charged me a small premium on top.  but he had run it overnight with his own GPU and HD plugged in with no problems.

I got the kit back and plugged it all in and ran memtest.  Everything was fine.  It ran three or four cycles without a hitch.  So I added everything else back and started to load Windows onto the spare drive again.  Glitch in the hard drive again.

I unplugged it all and ran memtest 86 once again and guess what?  Errors in test #6.

So at this point, after almost three weeks fiddling about in the evenings and weekends, tests running overnight etc I gave up.  I went out and bought myself a whole load of new kit which works.  So now I have:

* Mobo - Asus P5b deluxe (Intel 965 chipset)
* CPU - Intel Core 2 Duo 6600 Conroe
* RAM - 2Gb OCZ 6400 (800Mhz) DDR2
* Video - Asus EN8800GTS (nVidia 8800 GTS)
* PSU - Jeantech 600w Arctic Modular

All the rest is the same.

I would very much like to save/sell/recycle the old stuff but I've no idea at all what might work or what might not work anymore.  I have a horrid feeling that it must be the graphics card somehow upsetting the memory.  It makes sense as the cpu/mobo and memory were working fine overnight with this other chap.  It certainly isn't the HDD's which have been working flawlessly since.

Anyway, a cautionary tale for everyone and hopefully a good lesson for someone.Hmmm, well, I'm not sure what the lesson is here.  I'm not sure your "horrid feeling that it must be the graphics card somehow upsetting the memory" is adequately supported as a valid conclusion.  

I'm just wondering how much money you spent on computer components through the whole ordeal.  Clearly, it must have been many HUNDREDS of dollars.The lesson I think is that I changed quite a lot of bits and pieces which otherwise were the 'easy wins' and yet they were not.

It makes perfect sense that if your computer doesn't POST that there's a problem with the motherboard or power supply.  The power supply, by the way, is now fine.

I'm also confused about why the memory has failed.  This is ECC RAM which has had errors forced into it.  Makes no sense at all to me.

Maybe someone can take something away from this, and maybe someone can suggest further checks, but really I have posted it here in case it helps people.  I know that this sort of post might have made me feel a little better when I was checking the forums!

In terms of cash though, it has worked out at almost £1200 which is over US$2200 at current exchange rates.  The money 'wasted' on kit which didn't help is maybe £275 or over US$500.