Main thing I can say is don't be cheap. That ring is something she is going to wear the rest of her life. I splurged a bit when I bought the ring for my wife and she gets comments on it all the time as far as how much it sparkles and everything. Every time she gets a comment about it she gets real happy and if I'm with her I do too because I know that I picked it out.
However if money is definitely an issue, beware of what people will try to sell you. You can get diamonds that are very shiny and sparkly but are more expensive and ones that aren't quite so nice but are cheaper. But the cheaper ones just don't have the same affect when you look at them.
I woudl recommend going to a jeweler (NOT a chain in a mall or anything like that). Try to find a small-ish shop, maybe family owned, etc. They will give you the best service, the best advice, etc. If you happen to be in WI, go to this place:
http://www.lylehusardesigns.com
They are fabulous. That's where we got our rings and we've recommend them to everyone we know. Incredibly friendly, helpful, etc and they have great diamonds. You will pay a bit more but what you get is worth far more than the extra you might have to pay.