i got engaged last year and did a ton of research on buying a ring. it took me about 6 months of research before I actually purchased the ring.
i had gone to about 5 stores in my local malls and looked at all the different types. i also had to find out the type of cut, etc that my future wife wanted. i visited blue nile which is a great website. you can pick the cut, diamond, size and everything but just did not feel comfortable buying it online. they have a great return policy and whatnot but just couldnt do it.
so i configured the ring online through blue nile and then shopped around in the stores to find one that looked like it. i ultimately went with tiffanys....and before you say things like I overpaid for it, etc. i just felt like their diamonds sparkled over all the others. the salespeople were very friendly and they had free sizing, cleanings, etc. i just felt the most comfortable with them.
i ended up getting a .85 round cut diamond set in a channel with 10 diamonds on the side. i cant remember the clarity and color because its been a year but ill post a pic later of it....but it did run me about $9000 (which was my budget). i could have gotten something bigger from the nile but my fiancee is a tiny girl and honestly anything bigger would have looked terrible.
just do your homework, check out blue nile and then try to compare it with something in the stores....a buddy of mine went to the diamond district in nyc and got a ring on the cheap but mine looks 10x's better....so its all a matter of looking at it and deciding if you and her like it.