jojo, your best bet is to just go for it. I'd start going up in 5mhz increments. First core, then RAM. After each 5mhz, run 3Dmark2000 at least once. If the demo completes (doesn't lock up), and you don't see any artifacts, then go another 5mhz. As soon as it locks, or you do see artifacts, go back down the 5mhz, then go up again 1mhz. Keep doing that for both core and RAM.
Then after you think you have a good setting, try looping 3Dmark2000 or something else (Quake3?) for the night. In the morning if it's still running, you got yourself a working card.