GPU's are way more forgiving than CPU's are when it comes to temperatures. Once you start hitting 90, then you can start to worry a little bit, maybe look at some aftermarket cooling solutions. But even then, there's nothing strange about people running sustained temperatures over 100 degrees w/o problems. You can see
here that the recent Sparkle 8800 GT is running 111 degrees under load, and this is an OEM solution!
Bottom line, if your overclock isn't creating artifacts, you're good. With Rivatuner, you can boost the fan speed, but I've heard that it doesn't usually do much.