I'll just say my experience since I don't know what kinds of debt or expenses you have coming to you. I did it back in May so it wasn't that long ago.
I researched for about 2 weeks, asked coworkers, etc. You'll have to do your homework... any legitimate ring store should sit you down and explain that... along with the details of clarity, color, cut, etc. I think a standard healthy size is about 1 carat. That's all the carat-talk means... the diamond's size. Keep in mind, a 0.99 carat diamond will be SIGNIFICANTLY less cost to you than a 1.0 carat. It can be important for your girl to have 1.0+ carats, not for bragging reasons, but for the thought that it is actually over that "hump". I decided that I wanted at least that much - my fiance isn't materialistic at all.. and doesn't even usually wear jewelry, but I wanted it for her because it's not often you give someone something that means their life will change forever.
At any rate, I ended up with a 1.06 carat diamond. I bought it from a smaller, reputable store that actually manufactures their own rings, etc. locally here. Big stores (Kay's, Salomon's, etc.) WILL give you low prices, but the quality of the diamonds are comparitively less also. I'm not talking Tiffany's, I'm talking regular chain stores.
After choosing the diamond, you'll have to pick out the ring/setting. That was the easy part... no details except deciding gold or platinum. My price included a platinum setting & the round princess cut diamond - $5 Gs (of course they set the diamond in the ring for you). I comparison shopped and something similar with the same cut/clarity/color rating + setting goes for a couple grand more if you don't talk money with them.
What you can also do (instead of paying for a 1carat diamond straight up) is add sidestones (if you like that look) to make it go above 1 carat total. What a coworker of mine did was get a diamond that was about 0.75 carats and added sidestones ... that came to be around $4k for a total of 1.15 carats. It all depends on what's acceptable to you... money-wise and looks-wise. Hopefully she's dropped hints about that... my fiance didn't - it was a total shock to her, but she was happy with it. The other thing is the ring size... grab a ring of hers to bring with you when you are ready to buy.
Good luck to you!
EDIT>>> I wrote this when there were only 4 replies... now I feel like I was nuts to go for 1.0 carat... what gives? Aren't there any of you who went for 1.0 carat? The store I bought from showed me other people's purchases (they were real) and some of them were 1.36 carat, 1.15 carat, etc. I think anywhere between .5 and 1.5 carats would be an avg.