The best thing is to get her opinion. Let her go to one of those diamond sites. I'm presuming it's an engagement ring (at least our tradition is that the engagement ring has a diamond or the more expensive one and the wedding is generally a band to simbolize the unbreakable love)
http://www.diamonds.com allows you to design what you like. Make her have fun with it and choose 3 - then you choose what fits your budget, your tastes and what you feel suits her hand.
For a wedding band, I'd go for a simple band - you can have little diamonds on it but I think a wedding band should be simple.
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