I'd recommend a memory test (you will see this is a common theme on my posts). With RAM being so cheap nowdays, it tends to go through less QA than a lot of the other parts on the computer.
I just bought 2x512MB of ram for my system, and plugged it in, everything seemed to be going okay, but then it started crashing. My workstation that had gotten uptimes of over 45 days was not able to stay alive past two hours. So, I reseated the ram, and ran a full run of memtest+, everything came through. Booted up, and within half an hour, it froze again. Rebooted and immediately reran memtest+, errors showed up.
So go grab memtest+ at
www.memtest.org (it's an upgrade from memtest86) put your case in it's standard location (with the case on and everything), and let it run overnight. If you get even one error, it is one error to many.