Are you planning on going to Vista right away? I wouldn't worry about DX10 right now. It won't be available on any O/S other then Vista and I doubt there will be any DX10 exclusive games anytime soon.
As someone else had pointed out (in a different post), the first generation cards don't usually hold up to well when the games switch over to needing that DX version exclusively. Think about trying to play BF2 on a 9500Pro. The 9500Pro was one of the first DX9 cards and BF2 one of the first games to require a DX9 card.
Personally, I wouldn't spend too much on a Video card right now. Just get something that should hold you over 6-8 months.