I don't think 64C is a problem at all. Not sure what the max is, but thats not over it, not with how hot a lot of the newer GPU's run- lots I've seen crank in the 70's and 80's with no problems.
I'm just a bit wary, as the HD4650 isn't really a gamer card, curious if it can handle the stress. Especially with it being in such close proximity and confinement to my CPU/HDD/RAM.