A 660 would be a great upgrade. Assuming the rest of your system is up to the task (ie not CPU limited) then you would be looking at roughly double the fps at the same power consumption.
AnandTech - Bench - GPU12
If you want to go amd hd 7870 is a touch faster and around the same price, but uses a bit more power. Your call.
Really it all comes down to budget and what you expect of your system. What are the rest of your components?