Not all 6600GT's have temp monitors out of the box.
As mentioned by 905it don't just assume it's your GPU and then go and spend loads on a cooler that may not solve the problem atall.
Seems odd that it's just started to happen. Have you checked that your GPU fan is still working? Maybe even your CPU or RAM are overheating?
Dust build up over time will cause overheating, so are you sure your fans just aren't clogging up?
Have you changed any drivers lately?
So before you go out and start spending money just be sure you're sure what the problem is!