I don't know the card very well...I have a 7950GT, not sure how comparable they are, but my idle GPU temp is 47C.
64/79C could be typical...have you looked up temp specs for your particular brand/model?
Other things to check would be if the heatsink/fan is working, and if it is, maybe it's covered in a lot of dust. Or maybe your hs/fan just isn't that great. I bought a Zalman VGA cooler and it helps a lot.
Also, what's your computer case air flow like? You have enough fans in your comp to keep things cool?