I had a X800Pro modded to 16 pipes, it ran into the high to mid/high 70's most of the time over a period of a year.
Turely if you can keep it as cool as possible but if not just make sure its stable. I know my card was on the edge of unstable, but it ran (if it hit 83C it was game over and it locked or did a VPU recovery).
Lower is genreally better, if its a stock cooler with a mild OC, try to keep it on the lower side say 70's if it does go that high. If its an aftermarket cooler, I'd say 60's as a high tough.