7800gt is just about 90 bucks more than 6800GS, and it provides much better performance than 6800GS, and i don't think it is problem from one to get DX9 card, because dx10 games will not be released before a year from now, and 7800gt will able to run dx10 games, but it will run on dx9 mode
so i think it is better to get 7800GT