Did you uninstall the old onboard graphic drivers, before removing it?
if not, uninstall the old drivers with old card in, shut down, take card out, pop new one in, reboot, load windows & install new drivers (usually best to download the proper drivers, in otherwords newest ones from nVidia's website, before any of this, save it somewhere like the desktop or my documents folder, so its easy access after uninstall of old). see if that helps. I would also run a demo of 3DMark06 and see what happens. If you get through the entire bench without any problems then my guess would be the problem is software related and not your card. If you are still having problems afterward, try running Driver Cleaner Pro software and do a clean install of the latest nVidia display drivers.
http://www.nvidia.com/
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