Don't use your girlfriend's credit card for a cash advance to buy the ring
That's what my DH did, no kidding! But it was okay with me, and he knew that ahead of time. Sounds like a big red flag, but DH would never, ever have done it if he did not know that it was 100% okay. Our finances were combined at the time anyway. But I digress...
I knew the big question was coming and took the time to point out to DH every ring I liked
I didn't trust him for a minute to pick out something I would want to wear for the rest of my life. Definitely try to get some subtle hints, or go to a relative/friend (good suggestions from previous post). Have you talked about getting married or do you want this to be a complete surprise?
The diamond I have turned out to be flawed - DH didn't notice when he bought it - and that's probably why it was relatively inexpensive. You can only see the flaw in the right light. It bothered me for all of five minutes. To me, the commitment is important, not the vehicle used to convey it. I know others are different, but that's my $.02.
If your girlfriend likes antique jewelry, I have heard that there are good deals to be found at pawn shops or antique stores - not sure how you would check the authenticity though. If she likes the modern stuff, I guess you'll be heading to a jewelry store - I have no recommendations there. As far as how much to spend? I don't know that either! Personally I wouldn't go into debt for a ring, but my answer might have been different ten years ago...I guess you get a different perspective on things as you age. Good luck and congrats!