Ah, ok, just checking. Someone was running low-end GPUs in Xfire/SLI and disappointed with their performance about a year ago. At least you're using sufficiently powerful cards to see an advantage.
But I don't understand the reasoning--if you end up going 2x 660Ti's, you will pay more, unless you wait for the GTX 700 series come out, when you pick up an obsolete card at a reduced rate. Even then, it's a lot of waiting at half power (in terms of what you can afford) for a marginal amount of savings.
The cheapest 660Ti is about $250
, whereas the cheapest GTX 680 is $410
, which is about $90 cheaper than two GTX 660Tis *and* unlike SLI, the 680 will always perform much closer to its performance ceiling (SLI is never near 100% efficient in terms of scaling).
Short of a bargain deal on 660Ti's, you'd be better off grabbing a single GTX 680.--
Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety.