if you truly want her to absolutely 100% love her ring and totally surprise her, i wouldnt ask her about what kind of rings or go looking together at all. it'll get her thinking and she'll have it in the back of her mind that you might propose. women's intuition.
the best thing to do is to have an entirely different ring with which to propose to her, one that's more symbolic not cost. family heirloom from your side/her side, whatever. then you shop together for the "official" engagement ring, so she can hem and haw all day about which one she likes the most and which one she wants to flaunt to all her relatives and friends lol.