From what we see looking at GTX670 compared to GTX680, which was better than our estimations, we can safely predict that GTX660 will be faster than GTX580 for sure.
This will happens if Nvidia keeps the same specs from GTX670 apart from cores ofcourse.
So with 6/8 SMX enabled which means 6x192=1154 cores vs 1346 cores of GTX670 (7/8 enabled SMX), probably a little lower clock and if they use the same 2GB 256bit VRAM, its a win-win.
But I am reading rumors on the web that Nvidia is going to use 192bit and possible less amount of Vram.
Lets hope that the 1st scenario will come true and we will see an other magnificent card from Nvidia for around $300.
ps: On the other hand if they use cheaper memory we might see the price going to something like $250 :O.
thanks bro love the serenty prayer also