I'm quite fond of this PSU http://www.newegg.com/Product/Produc...82E16817153007
It isn't terribly expensive, and you can adjust the fan speeds so it's really rather quiet. My big 120mm fan drowns out the PSU completely!
Changing a psu isn't hard at all. You simply remove the old one, put in the new one, and plug all the connectors in the appropriate spots (one or 2 for the motherboard, one each for drives, maybe one for your video card). Just watch carefully when you take the old one out and you'll know right where to put the plugs for the new one. Basically all psu's supply the same voltages that every computer needs, so it's just a matter of getting it in the case and plugging everything in.