*if* you are going to overclock, then a highly overclocked 7850 will give you the most performance. The 7850 is something of a freak when it comes to overclocking, it's possible to get higher than 1100 mhz, though it requires a BIOS flash.
The 660 is better at stock speeds, and is a decent overclocker itself, though not as much as the 7850 The 2gb of RAM on that 660 will allow you more detailed textures without a significant loss in performance.
P.S. How much are the 2gb 7850's where you live?
Like i thought, 660 can be oc-ed but not as 7850, but still, i think it will do better than 7850 even if they are both overclocked to max.
its 285$ for sapphire 2GB. and thats significant difference. Average salary (per mount) in my country is 775$ so u can do the math