Couple of things off the bat.
1) don't get Tiffany's - the price is for the name and you can get much nicer (and cheaper) rings at a mom and pop jewelry store.
B) Read this:
http://www.lumeradiamonds.com/buying-diamonds/index
iii) I can give you a few recommendations on designers and diamond companies if you'd like. Though I don't know your budget