There was a pretty extensive thread about rings back when I was looking in 2010. I went with bluenile and was very satisfied. I got a way higher quality diamond and a nice ring/setting than I could ever get for the same price at a retail jeweler.
Here's a good read:
http://forum.calgarypuck.com/showthread.php?t=74543
http://forum.calgarypuck.com/showthread.php?t=96946
http://forum.calgarypuck.com/showthread.php?t=71248