Modern GPUs can take quite a punch in heat these days. What temperatures are you getting during idle and load?
As for OCing, I don't see any reason as to why it will be a bad idea to overclock it unless you do not know what you're doing. Assuming that you do, as long as your PSU is good enough and your card isn't running into excessive heat issues (and crash issues), then it should be fine to overclock.
Idle is about 43, and at FULL load in furmark it get's up to 85.