GPU advice upgrading 4890

System specs

750w psu
Phenom x4 955
4gbs ddr3 1600
GIGABYTE GA-MA790XT-UD4P

And 24 inch LCD monitor

Thinking its time to upgrade 4890, it still runs most things great but not flawless. Can play alot of the games on high still (with a few options turned down/off) at 1920x1080 but alot of newer games I need to lower it and goto 1680x1050. It just isn't what it was a few years ago.

What I'm wondering is what card/price would I be looking at to justify the cost of upgrading. Where it would really give me the best bang for my buck on a performance increase. I would like to be able to play things like Witcher 2 or bf3 on ultra or close instead of a so so high or good medium at 1920x1080.

Money is kind of tight but would be buying in the next few months for d3/gw2 mainly.