I don't know what kind of cooling it has and each card different. But if you do that and the temps at load is around 70's and it's running stable, then it should be fine. I guess you could even hit 80 degrees and it's probably still OK as long as it's stable.
In my experience though, raising GPU and memory clock never seems to bring significant real-life difference. Sure, you will get couple more points on your benchmark and feel bad-ass. But, it's doubtful that it will make a choppy game suddenly smooth or make your playable resolution much higher.
Also, overclocking will reduce the lifespan of your components. But, that's probably not a big deal since I'm sure 4850 will be considered slow in 2-3 years. Maybe even 1.