I'd venture that PSU doesn't supply enough +12v on the rails. Try upgrading to a 600-800watt good brand PSU and I'm fairly certain you'll notice the diffrence.

How many screens are you running? If you are running more than 1, Are you using the Horizontal span setting or the Dual view setting?
XP has issues with Dual view, this is fixed in Vista, but if you want to use XP then Horizontal span is really the only way to go.