I purchased an Antec TruePower 550 power supply today, but unfortunately that didn't solve the BSOD issue, so it looks like I'll be rebuilding the computer completely. As I've already got a decent server for more serious computing tasks, my aim is to do this as economically as I can. I've come up with the following list:

CPU AMD Athlon II x4 635
RAM 4Gb DDR3
Motherboard ASUS M4A78LT-M LE

I'll need to add a video card, as my two existing monitors only have VGA input (eg. the motherboard's DVI-D won't be suitable).