So how much does OC'ing cost in terms of lifetime? Seems to yield an extra 10% in hash rate, is it worth it?
It can be significantly more than 10%....
Stock I was at 292 MHs with poclbm. OC'd I'm at 392 MHs with phoenix.
That's some 30%!
Over-time-damage can be a gamble and often you can damage more than just your videocard. Cheap components on motherboards and PSUs will go up in smoke sometimes as well as getting a general life reduction.
Yeah, the 5870 does around 350 Mh/s without overclocking and 445Mh/s OC (maybe others get even more). If you lower the memory speed, keep the voltage low (I have 1'080 while maximum non-OC is 1'136) and the heat is low, why should it be a problem?