I wouldn't do it.
Unless your 6800's defective or overclocked too much, I wouldn't think that upgrading to a 7800 would do all that much, if anything at all. It's a more powerful GPU, which usually points at the idea of more heat... I'd try getting a can of air, and blowing out your computer parts (ridding of dust and crap), and then clearing any wires that may be getting in the way... If you upgrade anything, I'd spend a little less money on something like a new GPU HSF or a larger case for the air to circulate.
I just don't think that spending 500 bucks for a new GPU and only getting slightly better performance with, still, high temperatures is worth it. Make sure you're doing everything you can before spending that kind of money.