I don't think it's too hot until you experience problems running applications. That core can be broiling in there, but as long as you don't get artifacts, reboots, etc, then you shouldn't worry. It could also be that you have dust bunnies in there...I found some of those when the tech opened my XPS 2 up.
Depending on the ambient temperature, I get between 44-50C idling with my 6800 Ultra Go according to NVidia Temperature Settings. On my desktop 6800 vanilla, sitting here typing I get 50-51C with an ambient temperature of 39C according to NVidia Temperature Settings.
Note that the core slowdown threshold of a 6800 Ultra Go using Dell drivers is 101C, and the core slowdown threshold of my desktop 6800 vanilla is 127C. Your card would have to be defective to burn itself up--NVidia drivers automatically slow the core down to keep it from overheating.