I have owned 3 nVidia cards, a Ti4400, a 5950Ultra, and my current 2x (SLI) 8800GTX cards. I never had any heat issues w/ the Ti4400, but the 5950Ultra card would crap out at (or slightly below) 75C; the 8800GTXs between 78 and 85C.
The 5950 heat problem occurred when the fins of the heatsink got so clogged w/ dust that you couldn't get thru w/ a machete - keeping it clean eliminated any concern about heat ... duh ...
The 8800s, OTOH, run hot. Their huge size - double slots for each one and a hair under 11" long don't help at all w/ airflow in a midtower case...even w/ 6x80mm fans in the case (2 intake on the front, 2 exhast on the back, 1 intake on the side and 1 exhaust on the top). They idle at 60-63C.
If you are using/considering a top-end vid card, overheating is a problem that people are consistantly encountering. Be very aware of this issue, especially with 8800GTS and 8800GTX cards. The "designed to work up to 120C" mentioned above is seriously overstated. I've been spending a fair amount of time in the EVGA (my vid card manufacturer) forums...the temp limitations I mentioned are pretty standard in real life.
In order to try to give myself some more temperature headroom,
my new case will be arriving tomorrow:
Those are 250mm (9.8") fans on the side. It also has a 120mm intake fan on the front and a 120 exhaust fan on the back, as well as vents on the top and the other side. Since I'm not going to spend another $300 or so to water cool the vid cards, this will have to suffice.