Upgrading from a 660

Solution
If you play on a single monitor than a 2GB VRAM version will do, basically there are not much of a difference between both cards, the performance will be almost the same in real life and not in benchmark. GTX 760 is somewhat faster.

But you could go with a 4GB version card, wont bite you.
IMHO a 4Gb 760 is a waste of VRam and money. Even a 4Gb 770 is a waste unless you plan on running SLI and a large resolution. What resolution you going to be playing at? For 1080p or lower I'd either spend $30 and get a 2Gb 770 or save $50 and get a 2Gb 760.
 
If you play on a single monitor than a 2GB VRAM version will do, basically there are not much of a difference between both cards, the performance will be almost the same in real life and not in benchmark. GTX 760 is somewhat faster.

But you could go with a 4GB version card, wont bite you.
 
Solution


Then it makes no sense to upgrade. The gtx660 is a really fast gpu. The best you can do is to SLI, but I wouldn't even reccomend that, unless you have allot of space between the two cards.

Just wait for newer cards to come out. the gtx 760 is a 670, and do you really want to upgrade to (essentially) a 670?

Either wait a year or two more for newer stuff or buy another. But watch your temps.