correct. You can run 3dmark and run that a few times. Alot of games have built in benchmarks that you can run. Or just crank up a game and run it for 20 minutes. Usually if it is really unstable it will crash in a few minutes. I am not familiar with what people are getting in their overclocks, but you can also do some research and see what people are getting on theit 5770s and use that as a starting point rather than creaping up the clocks. You can sometimes at 50MHz on the GPU and start there.
Once you have a stable clock you may find it really isnt stable after an hour of gaming. It takes some time to get the highest rock solid stable clocks. In that case just crank it down 5mhz and keep going. I ususally dont go crazy on an OC. I usually clock it 10mhz down from the last unstable clock.