I have had a quite interesting experience overclocking my BFG GTX 275. I picked it up the other day, uninstalled and driver cleaned my ATI drivers in safe mode, proceeded to install the 186.18 WHQL driver as well as EVGA precision and Furmark, Techpowerup's GPU test and OCCT 3.1. I raised up the clocks, tested, raised, tested until I was at 735/1584/1291 (648/1440/1152 stock) and found that it was perfectly stable in Furmark for over 30 minutes straight (~90C temperature with auto fan settings).
Next, I tested it on the other two stress testers, and the clocks functioned fine in both. Then I gleefully launched TF2, joined a server and crashed. So I tried it again, and crashed again. For a while I was a little put off by this fact... how could TF2 stress the card worse than TORTURE tests? So I tried a few other games. Crysis crashed, Company of Heroes crashed, CoD4 crashed, everything crashed at clocks that were perfectly stable in torture testing.
So I decided to drop them back to stock to try it and found it was perfectly stable, not a driver issue or bad card or anything. Next I decided to use CoD4 and Mirror's Edge as my stress testing games. I kept finding setups that would function in Crysis and TF2, but for some reason not in those two games. Finally I settled on my shader clock being the big limiter as I couldn't bring it higher than 1506 in EVGA precision without crashes. I settled on 717/1506/1248.
Now, I still can't for the life of my understand why clocks that are stable in far more taxing tests than any game on the market are not stable in games as trivial as TF2 and CoD4. Anybody have any ideas? Power supply doesn't make sense, neither does a bad card, I tried multiple different driver versions, different OC utilities, nothing worked besides dropping them back.