DX10 Question

People keep saying to hold out for DX10 cards instead of buying a high end GPU now. If I don't plan on switching to Vista for another year or so how much of a gain would I get with a DX10 card? I was under the impression that DX10 was vista only. Will the performance of a DX10 card used with XP and DX 9 help at all? I'm a little confused as you can tell.

First of all, the cards aren't special in any way because of full DX10 support. They will be much more powerful, faster, and will handle today's games much better. Vista and DX10 have nothing to do with the fact the cards will be a crapload better, faster than current cards.