I had similar issues after upgrading to Vista 64 and upgrading my hardware. I replaced my new Ram chips with another brand and replaced my power supply, but the issues remained. As long as I wasn't gaming, the PC was perfectly stable for days. I decided that maybe my video card was the problem, so to help with cooling I upgraded my case. Nothing changed, so I started looking at forums and reviews and found that my 9800GT video card had a really low factory fan speed setting to keep it quieter. There were a lot of complaints about the card overheating. I installed Rivatuner and moved my fan speed up from 30% to 50%. The problem completely vanished.

I hope your problem is similar because it's an easy fix for something so frustrating. Good Luck!