video card says it is too hot

[surfdoc]

I have a Dell dimention 8400 and an nvidia 256MB geforce 6800 video card. When I have a lot going on with multitabling, I am getting an error message that says the video card is getting too hot and the video output may suffer. If I keep going it eventually shuts off. I let it cool but it keeps happening. I don't want to fry it obviously.

When I open the unit up it looks like the fan is running okay. I am pretty computer noobish so any help appreciated. Thanks.