I recently went through the same process. I previously used an Ultra PSU that I got free after rebate a while back, but finally listened to all of those comments about the importance of the PSU and that a substantial investment in other parts was riding on it.
The first step is to use a calculator like the one at http://www.extreme.outervision.com/
(there is a free Lite version or it costs just $1.99 for the pro version that gives you the amperage needed as well). I ended up getting the Corsair 520HX from buy.com for $89 after a $10 rebate. It got top marks from silentpc: http://www.silentpcreview.com/article699-page1.html
and has a very high efficiency rating. I can atest to the fact that it is extremely quiet and has modular cabling, which was a big plus. The only negative thing I can say about it (and this is being extremely picky), is that the cables with sata connectors only have two connectors on each, meaning that for my three hard drives I had to run two cables to that part of my case. From the manual, it appears that the 620w model has a cable with three connectors on it, which would have been more convenient.