My 690 is now 4 years old, and has not missed a beat.
When I bought it , it was the fastest card that money could buy, from memory it was $1200
so I have been very apprehensive about overclocking it.
Had a "play" with some conservative numbers and playing Assetto Corsa last night and got this
stock
small increase, 50 on clock, 200 on memory
BTW: stock, I was playing for about 30 minutes, second one I only drove a few laps then went back to default settings, cos scared