a lot depends on your case too. I have the MSI gtx660ti PE 2G edition and I can barely hear it at all. I have the hfx full tower case and even with all the fans in that i can barely tell that my PC is on. I O/C'd my vid card last night and in doing so cranked up the fan speed to 60% and I could then hear it but at normal speed in a similar case I doubt you'd hear anything.
I start to hear my GTS 450 at around 55% fan speed, I could probably get away with setting lower fan speed profiles since it only gets to about 56c but since I'm gaming when it does reach thoughs temps i'm not too bothered.