OK, an update...
I flashed the BIOS to the latest version (which has only been out two weeks I think), reinstalled Windows, did all the updates and put my soundcard & graphics card back in and (fingers crossed) it's been fine so far - been up for hours now with no problems.
I just hope this is all ok now because going back to the A8V/FX53 combo made me realise just how fast it is compared to my XP3200/Asus A7N8X.
FYI - the graphics card is not the problem, I tried an MX440 in it and it still locked up.
That was before I reinstalled & flashed the BIOS.
Please god, let it be ok now!!!