Overclocking an HD 4650 would not yield much more performance for modern games. If you have 12FPS in a game, an 30% overclock will only give you 12+0.3*12 = 15.6FPS.
However, overclocking modern cards will yield a lot more performance. Consider you have 70FPS in a game, that 30% overclock will give you an additional 21FPS, which adds up to 91FPS.
But if you do really want to overclock, safe temps are below 90C for graphic cards. Above 80C you might be shortening the GPU's life, but it is not significant.