I'd probably go for the 5850 for the $250.00 price range your looking for. This should be slightly better than a 4870 and if you O.C. , it should be much better. I wasn't able to find too many benchmarks for the 5850 but I did find them. Most benchmarks were for the 5870. Good luck on you search. I have almost an identical sysyem as you and was thinking the same thing to upgrade my gpu.
I have a antec 900
asus p5qe
ocz 700
e8500 stock
4 gb 8500 memory
640 hd
4870 asus 512 mb limited I bought for 300.00 last year.
let me know what you decided to do.. Just curious.....