Unless its $75 or lower it's a bad buy second hand. You can find 7950/R9 280s for $100 and they have 3GB plus the grunt to actually use it. Quite a bit faster than the 660.
I wouldn't get a 660, at all. They're not very fast and I think you'll end up replacing it sooner rather than later. A $100 R9 280 / HD7950 is a really good buy. They overclock quite well too, not unusual to get another 25% FPS out of it at the end of the day.