camocrazy:
-Make sure that the video card is properly seated. and you have installed all the latest drivers and patches of this card from?
http://www.nvidia.com/object/winvista_x86_163.75.html
try to play games now and if the problem still exist then do:
-At this stage I would recommend just to blame the card's software (drivers) uninstall all drivers and software partinent to this video card. Restart the machine and play the game for a while, see if your monitor crashes. If still does crashes then its the OS.
-Simply uninstall the graphics card from the PC and check if the perfomance of games effect your video. If it still does effect then there is a problem with OS.
Finally I would suggest to simply install the card install the latest patches and drivers from the website of the manufacturer and restart your machine if it works then we can blame faulty driver and software.
-Also please check the manufacturers website of any known issues with this particular card with Vista.
Hope that helped!