I actually just had to mod out my brothers for that very reason. What brand is it? Sometimes they come with a cooler on it, but it isn't efficient enough.
Or, better yet...try this. Take off the side of your case. Now get a big ass floor fan and crank it up to high. Place the fan so that it blows into your case. Now try to run a 3d application and see if it lasts any longer.
its not the heat issue. i can stick one in my system dead cold and the first thing i do is run a 3-d app. (this whole process taking approx. 2 min) and itll stick glitch up. im pretty sure its not the heat. any other ideas