See this: GPU Dictionary: Understanding GPU & Video Card Specs - Gamer's Nexus | Gaming Hardware, Culture, Features, and PC Builds
I would give the 7850 overall edge over the 660 also, again that memory interface is why, and I preach go for more when comparing cards, if you have the PSU juice for them/budget.
The memory interface injunction with the memory bandwidth is what makes or breaks the real quality of a video card. Article quote: A smaller interface can be compensated for by faster memory clock speeds or different types of memory.
But if it's close, the bandwidth interface (size) gives the edge to the better one, it's in use ALL the time.
My 7950 is a 384-bit card, if it was half (192) yeah it would be mediocre. I finally took the leap to top tier, first time ever, and this sucker had better last a while. muhaha
[It should, I behave, I still have my 5850, 8800GTS, and 7950GT all still in working order]
Your 660 is probably basically a crippled version of the 384-bit top tier cards.
Bring on the 32 core CPU with multi-CPU motherboards and 2048-bit bandwidth video cards, we have suffered enough!