Might be out of your price range, but I have a PC Power and Cooling "400 Silencer". Supposed to be quiet right? Wrong. PCPC btw is the undisputed champ of PSU's but they carry a premium.
So a couple weeks ago I replaced the stock 80mm fan with one of my own, a very quiet, low-cfm Panaflo. Now the thing is next to silent. It rules.
I've opened a couple different psu's and not all are the same. On my PCPC, it had a nice little "standard" two-pin connection, like a motherboard header. Others however seem to be soldered to the psu's circuitry. So depending on your skill level stuff like that might not be an option.
From reviews I've read it seems most modern psu's are noisy, and the only way to fix it is to change out fans. Someone point out a nice and truly quiet psu if I'm wrong on this.