So, I ended up going for a Palit GTX275 I spotted that was just slightly over what I intended spending. Got that hooked up with a new motherboard and processor (AMD Phenom 9650 Quad-Core Processor), 4 GB ram and running Windows 7 64-bit.
But now I have a big problem - when I play any intensive games like Crysis Warhead or Prototype, my system crashes, just reboots after a few seconds to a few mins.
Any ideas what it might be? I have a 550w PSU. I also tried it with Vista 32 bit but same problem. It's not OC'd or anything, and I've whacked the GPU fan up to 100% and it's only hitting about 80 degrees before the crash, so don't think it's a heat problem.
Driving me nuts cos I was well looking forward to getting stuck in to some stuff!