Just recently I upgraded my graphics card to an Asus GTX 550 ti .
When I play games after almost 15-20 minutes the game suddenly freezes and its process closes! Nothing but just the game process is affected and my OS runs normally. I use task manager then to close the "not responding" game.
I just then check my GPU's temperature (using Asus SmartDoctor) and it is usually around 60-64 degrees while the fan is spinning at 4200 RPM at that temp.
Games so far played and affected with this problem :
I had that mb. chgeck that your ram is 800 speed.the mb has an issue with 1066 or faster ram under load. with your system it could be that it running out of ram under load and it crashing. the other issue is under the games is you have the video setting set to high and the gpu running out of ram or your causing an issue when the gpu and ram try to swap data out of the gpu frame buffer. i would see if you can toss in another 4g of ram and then see if the games lock up if they do try turning down the eye candy.