what is your gaming resolution? what kind of frame rates are acceptable to you? if you do willing to turn down the setting down to medium so you can get minimum of 30 frame rates per second then 2 660s will going to last you for quite sometime even on 1080p. my bet? maybe i'd say that such setup with such requirement may last you more than 3 years for majority of the games available.
ill will be playing at 1080p, and anything above 40 frames is acceptable to me. ill will probly upgrade in 1.5-2 years anyway but i just wanted to be sure it would last and be not completely outdated next generation