I believe that the 8800 can run over 100C without any big problems, but that will generate a lot of heat for the rest of your system and it probably isn't healthy in the long run so it's recommended to try to keep it under 70C.
So getting a couple of extra case fans, replace the 8800 fan with a better one (Zalman etc.) or even better, add water cooling are highly recommended when OCing it..
Like this one from Zalman:
http://www.zalman.com/ENG/product/Pr...ad.asp?idx=283
