Your card maybe over heating (do you have good cooling for your computer.)
Or it could be a driver issue. What drivers are you using at the moment.
I could of said check for patches for your games but because you said it happnes with all games it is more than likely Graphic acrd ussue/heat/drivers.
Here is something for you to look at and gives soem help.
http://www.computerhope.com/issues/ch000224.htm
I am also reading it could be a memory issue.
Have you upgraded the amount of memory you now have, did games play well before you upped your memory to 1 gig?
If you are comfortable with opening your computer take a stick out and just leave 512 in then switch comp on and try playing a game see what happenes.
I take it you have 2x 512 sticks.
After what i have read (i am reading while answering) it does look like memory is the most likely reason (but not guarenteed)
You should type General Protection Fault into google and spend some time reading through all the links discussing it, you will get a good idea of all the different reasons and you can start working through the more likely ones.
Good luck.