There are several variables that we do not know besides the power supply.
Motherboard Chipset? eq. nVidia 690, Intel X 38, etc.
nVidia Driver Version?
Recently a friend bought a ATI 4830 to go into his aging system (P4 2.8) Turns out that he got higher FPS using my old 1900pro. This is because his motherboard 925xe chipset is just too old to supply the bandwidth the new card was wanting.
Your power supply may be causing a problem, however in my experience I tend to notice high temps when this is the case. If you are using windows vista then it may well be your power supply because a graphics problem tends to not lockup or bluescreen your computer.
Hope this info helps some from a hardware perspective.
From the driver perspective you can download and install driver cleaner pro. Then uninstall your video drivers, reboot then run driver cleaner pro and clean out the rest of the driver junk and then reinstall the latest drivers from nVidia's website and see if that helps.