You should definately give her a ring, but I think most women like to pick it out. Some places will let you buy one and you can return it within 30 days if it is not the style she wanted, as long as you get another one from them.
I did that for my last ex, and she upgraded to one that cost me about four months pay (yeah, I was
)
Ask the jewelers if that's an option. You never know, you might pick the right one, but it's nice to know she can return it if she wants to.