If you are going with a diamond, another vote for blue nile. You can get a quality stone and setting for a lot less than a brick and mortar retailer. Or, put another way, for the same cash output, get a way nicer quality ring. Plus, you'll get a reputable grader... GIA is way more discerning than IGI, for instance. So a very good cut VS2 as graded by IGI might be labeled only a good cut SI
See if she can't be talked into CZ. Don't go for moissanite, IMO, they look gaudy compared to diamonds. Even my untrained eye can point them out. They often have a subtle yellow or greenish cast, and they are doubly refractive, so they look over-the top rainbow-y. Pretty enough, but not diamond like. A well cut CZ is far more comparable in optical qualities to a diamond.
FWIW, I asked for CZ, DH agreed, but at the end of the day he decided on his own to go with a real diamond and went with blue nile. I am really fond of my ring as it is, but I would have been just as happy with a CZ.
I have pretty small fingers (ring is size 4 and make could be a hair smaller yet). So my .65 carat stone is a nice size on my hand, and like others mentioned, a good price because of it's weird in-between size (since it seems diamond weight milestones are at quarters of carats, a 2/3rd carat is kind of unusual.) DH got what I think is a very nice quality stone (maybe a little nicer than needed...). Specs are: H color, VS1 clarity, ideal cut, with excellent polish and symmetry. He could have gone to I color, VS2 clarity, but I do appreciate the ideal cut. You can even get CZs in an H or I color... I prefer them over colorless as colorless looks a bit "cold" somehow. In an 18 kt white gold setting with .20 carats of pave diamonds on the band, it was just under $2800 otd. (I only know because he had gotten a 4.5 and we sent it back to get sized down, and the customer service lady told me exactly home much to ensure the package for.) But remember, gold prices were just starting to rise at the time, which was early 2007.
My only complaint about blue nile is that they don't have less than 18kt gold, or at least they didn't when DH proposed in 2007.