I'm not really sure what your budget is, but the 8800 line of cards from Nvidia just came out and they support DX10, so if you want to wait a bit and save up some cash, you'll 'future-proof' yourself if you go that route. It's a couple hundred dollars more for the cheaper 8800GTS:
http://www.newegg.com/Product/Produc...SubCategory=48
You'll be ready for vista when it comes, I'm positive SHIV will have some patch in the future giving it DX10 functionality...I'm building a new system when Vista comes out, they'll probably have something better than the 8800GTX when Vista does show it's head, so I'm being patient right now...