Just got my BFG GTX260 OC (192 version), was at stock speeds 590/999/1296, i overclocked to 650/1210/1400 but it was getting close to 85-90 degrees so i backed down to 642/1202/1400 and the MAX i have seen it go is 81 degrees when running Furmark at 1600x1200 with 16xAA and Extreme Burning Mode enabled. A bit overkill to be sure, but I wanted to ENSURE stability over all else haha.
Mind you this is all with no adjustments to the fan speeds, highest usage the fan has seen is 54%. I use Rivatuner so I need to make a few fan speed profiles and surely I could push the card farther, but honestly there is no need. I'm happy with my current overclock.
EVGA 750i SLi mobo
Core 2 Duo E7200 (stock 2.53GHz, OC to 3.1GHz on stock cooling. Vendetta coming soon!
Single BFG GTX 260 (192) (OC to 642 core/1202 memory/1400 shaders)
2GB OCZ Gold DDR2-800 RAM
3DMark 06 score of 13,903 (limited by my CPU big time, but its a budget system so what the hell!