I just bought a Geforce 6600 GT and the card runs good. For about 15 minutes that is ;(. In normal conditions like browsing the internett and such it works like it should. But when i play a high demanding game for about 15 minutes the screen freezes and goes black, secounds before it freezes i see black dots on the screen. Any help would be greatly apriciated
I got a 400Wats power supply btw

i'd suspect a heating problem. let's hear some more detailed information on your computer's operating temps (whatever you can find via software/bios) and your case/ventilation configuration.

download and run a prime95 torture test, selecting the "in place fft" option. let us know how long the machine runs after initiating said test. if it runs longer than 30 minutes or so without crashing your system, stop the test. i advise repeating the tests several times to eliminate the chances of a random error not related to your problem misleading you.