I've been playing with a Cooler Master Gemini II with one Silverstone FN121 fan this evening-- on an E6750, I'm having a very hard time getting it to spin up to an audible volume, using it on a board with a "Smart Fan" setup (Gigabyte P35-DS3L). At idle, it drops to like 600rpm and at load, it's sitting at 883RPM.
The nice thing about the Gemini II is, because it was underwhelming in the "Compete with anything from Thermalright or a big Scythe or a Zalman 9x00" class, it sells very cheaply. For a while, they had super-agressive rebates on it, where you could get it for like $2 or $3 after rebate. Not right now, but I got mine at $3 or so.
I've heard claims anything below 1200rpm is inaudible on a 120mm fan.
My present case has one 120mm exhaust, an Antec SP-500 PSU (the rear fan only runs on presence of heat), and the loudest things are the drives spinning up and the tick of the hours-of-use meter on the front panel.