I'm also thinking about buying a quiet aftermarket cooler for my i5-750. I just finished building my Hackintosh last night (specs are in my signature). I'm using the stock Intel cooler and noticed that the CPU fan noise is pretty loud, even when the computer is "idling". It also seems to constantly vary between two speed settings, even at idle. I'm not doing any overclocking.
I installed the iStat nano widget, and it shows both CPU temps are around 40-42C when the computer is idling. Is this what I should expect with the stock cooler? This is installed in a CoolerMaster Elite 341 case that has a single 120mm chassis fan in the back, and the PSU is a Corsair 550VX that's supposed to be very quiet. Room air temperature was fairly cool at the time.
Maybe I should try reinstalling the stock cooler with new thermal compound? I installed it with the stock thermal paste that was pre-applied by Intel. I've got a tube of Arctic Silver 5. Should I really be seeing 42C at idle on a stock i5-750?