I like more 4pin pwm fans, cause you can control them via bios/software, some mobos can control 3pins via voltage (not all), but fans like the Scythe PWM can drop down to 300rpm, which are almost silent, but at this rpms they also dont move much air, but the good thing is this fans can be ramp up to 1600, its up to you, so you know they will work both very low and high.
I been playing with the mugen 3 on the weekend, although the server isnt working great (i think my psu is having issues at cold boot, arleady have CP850 on the way), i was able to toy around with the setup for some hours, including some prime95 stress and the cpu only goes up to 800rpm with full load on a i3 2120, netting a temp of 40C, still a lot of room on the temps to lower the rpms more, my guess is you can let it be at 300rpm at idle still netting around 35C, and let it ramp up to 500rpm on load and still have around 55C (on full load, normal task should still be much lower)... this is a guess, so take it with a grain of salt. With an i5 2500k it should have a little higher temps due to more cores and higher tdp cpu, but i think you should be able to run it below 800rpm easy, this is the good thing about this fans you can tweak around until you have a good balance of cooling/noise.
I believe there are 2 slipstreams pwm fans, the one on the mugen runs 300/1600rpm and there is a slower one that runs 200/1300rpm, Scythe Slip Stream 120mm x 25mm PWM Mid Speed Fan - (SY1225SL12LM-P) $9.95
, in case you are interested on swapping or adding case fans. In case the mobo dont have many PWM fan heathers, you can use something like Akasa SMART PWM Fan Cable Adapter - Black Braiding for 5 Fans $7.95
, with one 4pin pwm mobo fan heather you can run 5 pwm fans, and not stress the mobo as it takes the current from the a 4pin molex, just uses the mobo for the rpm and pwm signal.