Don't shop for a "DEAL"... Find a ring SHE LOVES... BE DONE WITH IT... Seriously.
No Really, don't waste any time trying to "Bargain SHOP", not if your girl is all wrapped up in having some huge freaking chunk of bling... (You poor bastard if she is)...
One other option though is having your rings custom made. Get something truly different from a local jeweler. Doing it this way you might be able to avoid the 400-700% markup.
But that's what I would do, find a something truly unique and special. Otherwise you are just getting one of 10K or so...
Also, avoid 14K gold, in much of the world you cannot even legally call it gold at 14K. 18K is a pretty good, 22k is about the best you'll find, Platinum is a good choice for bigger stones as it is stronger than hell.
But yeah, take your time, make sure she loves it, and then just buy the thing.
Cheers.
Oh and I'm not truly a Diamond expert, but I have my BFA in Metalsmithing and Jewelery. There are some very well respected small jewelers in the Edmonds WA area... Tom Shane isn't a bad choice nor is Jerods though of course you pay for a lot of their nationwide advertising. So I'd find a trusted Mom and Pop... Get something really special.