I do pretty well with my 9800GTX overclocked.
I have my 6000+ overclocked a little too. Im pretty sure its not bottle necking the gfx card. If it is, I cant tell with the bench marks I have run.
I would say, go for a 250 (same as a 9800gtx+) or a 260. As for the Ati side of things, I cant really comment.