i went from a 4850 to a 460 768mb... the difference was pretty good, especially if you overclock. I have my 460 from 675 to 920mhz! , I am very close to gtx 470 benchmarks on metro 2033 at 1680by1050 (on my 20" monitor), and on 3dmark 11 I am also close to the 470 and 6870.
If you have the money though the card i recommend you to buy is the 6850, which I hear can be over clocked to 6870 levels. If you are short on cash, the 460 768mb is very good, and noticeably better than the 5770, 4850, 5750, gts 450 despite what the toms hardware charts say.