9800gt Lock ups

[hummer11]

After about 1 hour-2 houres of playin COD4 the game will freeze and lock up. I usually have to reset the system but once it fixed its self. when it freezes the GPU seems to go of and the screen goes black and i can still hear the sound. vista showes no error report. could it be overheating? i think the fan was on 50%

MSI K9N2 750a SLI
8GB OCZ SLI PC-6400
Phenom 9750 2.4 GHZ
450W PSU only 25a on 12v could that be the problem?

I just ordered another 9800gt and a new sli psu.