Yeah, GTS 250 is essentially a 9800gtx+ that uses less power/heat. As shadow said though, look out for good prices on 4870/50 cards as that is where I would think the best 'bang for yer buck' is going to be.
i have used ati cards for prolly the last 5 years my last nvidia card was an old fx5200 before i got a 9600pro and 9800xt. over the past couple year though i havent been too happy with ati software and wanted to try and change things up a little right now i currently have a 4650
ATI software has vastly improved; you will not be disappointed. Older cards like the 9600/9800 don't have very good drivers, and most likely never will.