the 7900GS is a very powerful card and a great choice. Definitely worth getting.
As for DX10, no. You'll be able to play the new DX10 games when they come out, but only on DX9. To run in DX10 you'd need a card especially designed for DX10 and so far only the 8800 series do that.
But there are no DX10 games out yet, and the prices are still pretty insane so no need to rush out and buy one yet.