Your best bet is your local wholesaler. If that wholesaler has a good goldsmith then you're rockin'. Make sure you educate yourself on the five c's - carat size, color, cut, clarity, and cost. I always add the 'cost' because the diamond marketing industry doesn't want you to think about cost as an issue because they want you to spend as much money as you can. Sneaky bastards.
[l]http://www.wam.umd.edu/~sek/wedding/diamonds.txt[/l]
is a great resource that has been around for awhile.
I've written a ton about this subject and you can look it up over at
www.dvdtalk.com 'other' forum. Same username. This should clear things up for you.
Bottom line: I know it sounds kinda ... well, selfish but always buy something nice, flattering and impressive but NOT gaudy. 95% of women will say that "I don't need that size of rock blah, blah, blah" ... but you'd best err on the side of caution because she will wear it for the rest of her life.
Seriously.
So, if her hands and fingers are quite slim (piano fingers they say) then get something that looks good but impressive because as much as they say it DOES matter when other women say to them "That's a gorgeous ring!" Deep down your wife will thank you.
I can't emphasize this enough. Again, I know it sounds possibly cheesy. People like to get complimented on or at least have the option to demur or defer the compliment with a "Thanks" or "I didn't want my husband to get me something like this ... it's too big!" They all love to say this ... so give her the reason to do so. An average rock for an average ring will get an average response ... which is nothing.
As for Blue Nile, their rocks seem alright but it's hard to say without holding that rock in your hand with a 10x loupe in your eye. Sometimes it's hard to fathom the table and depth ratios over the 'net. You won't know what an ideal cut shape is until you go to a dealer and see one for yourself. Most dealer/wholesalers will match or beat Blue Nile anyways so don't sweat it. Plus, buying from a local will get you free polishing for the rest of your life which is actually nothing to sneeze about.
But also check out shanks (ie: rings w/o rock) and ideas over at my favorite place
[l]www.r8w.com[/l]
They have some nice shanks and settings there. If only to give you an idea.
Plus, going to a dealer/wholesaler with their own goldsmiths will allow you to do a custom job since pretty much all rings in regular stores in malls and such use stamps for their shanks. Uggh
Good luck.