Thanks for the responses guys. Haven't had time to check out that memtest program yet. Gonna try that, but can I also burn that program on a CD? Even though I also suspect it's a problem with my power supply it's good to find out.

You guys think a 400 or 450 watt supply could handle my system?

I didn't have much problems with the PC in the past couple of days luckily. I was even at one point hoping the problem was solved by one of the latest windows updates... but it still locked up after that. Would've been too easy I guess...