I'd crank it up until just before the noise gets annoying, and set that as the idle speed. Then pick a higher speed you won't notice during the noise and action of a game, and have that as your load speed. Throw in a setting to put the fan to 100% when it gets near overheating (you can set it to throttle the clocks back as well). You can also toss in a few intermediate settings if you feel like it.
Oh, and no, this will not kill your fan. OK, technically it will, but we're talking "dies after 9 years" vs "dies after 20 years" here. I don't know what it is about PWM fans that make people think they can't be run anywhere near full speed. Somehow running your case fans at full power constantly won't ever kill them, but setting a variable speed for full speed and...OMG TURN IT DOWN IT WILL ASPLODE!!!!!!!!!!!!1111
PWM does not make a fan run faster than normal under load, it makes it run slower than normal during idle. "Normal" is full speed all the time. This is excessive and annoying for most fans, so we have devices to slow them down when not needed. Some laptop and GPU makers go a little overboard setting these, hence the reason for programs like RivaTuner, SpeedFan, and smcFanControl(Mac).
These are the exact same cards I have, two in SLI, and I have my setting to take the fans to full bore on Windows boot. They run fairly warm if you don't do that, as the fans spin at about 30%. I've found that the dynamic scaling does not work very well with these cards, so even under load, the fans stay at idle speed; so I'd just set them high and rock out. In terms of noise, I'd say it's "lounder than idle", which isn't saying much; they probably together sound like a 3" case fan. Go all out, all the time, from my experience with those cards, you probably won't see temps higher than 50C (in a decent case.)