I get 53 degrees with the 8800GTX underclocked as much as possible in nTune and idle, and I have 5 case fans. I guess that's about the minimum temp you'll ever see for a 8800GTX with the stock cooler. With the default clocks (or a bit higher, since I have a factory overclocked card) I get 80 degrees, and it still works fine.
What I'd suggest is to download nTune (from nVidia's site), install it, then right click the nVidia Settings icon in the Windows taskbar, select nVidia Control Panel, then use that program to configure the card so that the fan runs on automatic. That will make it work like mine - the fan works harder when you're playing 3D games and the temperature stays reasonable.