What model PSU? Sometimes if a Power supply cannot give enough power to a card, you will get this sort of situation where games that take less GPU power will run but once you put the card under more load, they freeze up. Because of the particulars you described, this is the first thing i would look at.
System Freezes can also sometimes be the cause of faulty ram. If you run http://hcidesign.com/memtest/download.html
for several passes and come back with any errors, it means you have a bad stick of ram. You would need to test each stick 1 at a time to find which one it is, or better yet, just replace the whole kit with a new kit.
How did you determine you have a bad GPU? Just out of curiousity.