i have a 32 db fan in my case (the panaflo 80mm 39 cfm fan), and it's just right for me

not too loud, and leaves my cpu niiiiice and cool (with the help of an alpha pal8045).
as far as the connectors go, you need to look at the wattage for the fans you are gonna buy. most motherboard headers can take at most 4 watts, IIRC. the benefit is that you will be able to monitor the rpm of the fan. the benefit to using the 4 pin connectors is that you put less strain on the motherboard. the best of both worlds is to buy (or build) adapters that split the power connector of the fan. the power leads go to the power supply, and the rpm monitoring lead goes to the motherboard.