I have an Asus 660ti dc20-2Gb that I've over clocked using Asus tweak. Currently it beats a stock Gigabyte G1 gtx960 in passmark by 1-2k in every category except tessellation, on stock cooling and still only maxes out at 78°C at 1080p presets. At stock settings I was getting 60+fps on heavily modded skyrim and ac3 at ultra settings with AA/AF 4x/8x on dual 1080p monitors.
Dont sell the 660 short, for a mid grade card it's still decently powerful.
R9 270x and up have 3Gb versions, as do 660ti and up uand that's about as much as you'll need for 1080p. 2Gb is quite enough for a standard 60Hz 1080p monitor. 4Gb on mid range cards is overkill, and was only a consideration for those running sli/CF on multiple monitors using eyefinity or nvidia surround. For a single gpu/monitor it's almost pointless and honestly not worth the extra $ over a 2-3Gb card.