Windows 7 freezing during video games

Hi

Whenever i play certain games (left 4 dead 2, mass effect 2, dragon age) my game and computer will freeze up. I cant even Ctrl alt Del out of it and i have to end up restarting my computer. I have tried reinstalling windows, updating my video card driver, and updating my Bios. The motherboard manual does not list my current Ram i am am using as compatible, could that be the problem?

Windows 7 64 bit,
AMD Phenom(tm) II X4 965 Processor,
Nvida 9800 gt,
Asus M4a78T-E motherboard
650 watt psu
Corsair Xms3 DDr3 4 gb

Pls help

What happens when your computer freezes, do you get any blue screens or error messages, or does it just keep the game image on the screen?

If you have ruled out a software problem then it could very easily be your hardware. Have you overclocked and is your system cooling adequate?

Just keeps the image of the game frozen. Also i havnt OCed anything on my system so that shouldnt be the problem. i installed an older video card of mine and everything seemed to work fine (except for the fact the card is old and crappy). Also what should the normal running temp of a video card be?

That does make it sounds like the graphics card has a large part to do with it then. Are you able to try it in another machine to see if there is a hardware fault?

The max GPU temperature is supposed to be 105 degrees C and will use up to 200W power.

What make/model PSU do you have?

I got an antec neopower 650 watt