I see two routes... either case, pick a place with a great exchange policy. I like Shane Co myself for jewelry purchases. The prices were good, and I was treated wonderfully. Their unconditional lifetime guarantee is great too. Be careful of lifetime guarantees with requirements... if you miss a cleaning by a day, they can void it.
That said...
1) Pick out a plain solitaire ring for the surprise. Then go shopping after the fact so she can pick the ring of her dreams.
2) If you think you can peg what she wants, then go for it. I agree that the quality of the gem is more important than the size. Go for something unique. NO woman wants to see her ring on another woman.
