When was the last time the operating system was reinstalled? Anytime i make hardware changes (especially when changing from nvidia to ati video cards ) i reinstall XP. I know you havent mentioned what brand your old video card was , so i added that just incase.
Heat , IRQ sharing and drivers could also be to blame , make sure to use driver cleaner when changing drivers , and don't be afraid to try different ones . Pc power /cooling psu's are top of the line so i wouldn't think it would be a psu issue , but... you could have just gotten a bad card . Good luck