I'm going to throw a line out for some opinions. If you want, skip the TL;DR and state which option (in the table) you'd go with.
Here's what's going on. I built a Z68, 2500K, 16GB DDR3 rig in August last year with the idea of stepping up from a GTX 570 (note, 2.5GB) to a GTX 670 or getting a second GTX 570 sometime in 2012.
Well, it's that time and the price of the GTX 570 in question has only dropped by about $80, which isn't what I really hoped for.
I know the routine. A single GTX 670, for $100 more than a second GTX 570, will outperform the GTX 570 by roughly 30%, but two GTX 570's will surpass a single GTX 670, but at the cost of power usage, heat footprint, SLI glitchiness and the relatively insignificant 8x + 8x limitation of the Z68. Moving to the GTX 670, I doubt the move from the current 2.5GB to 2.0 GB will make a serious impact.
The GTX 680's are $100 more than the GTX 670's, don't perform much better and have the same amount of memory, so aren't an option. AMD is a remote option, but after persistent driver issues with previous cards, I'd rather avoid them, unfair or not.
Option # Card(s) Model Cost Plus 1 (1x) EVGA GTX 570 2.5GB 025-P3-1579-AR $0 EVGA GTX 560 Ti 2 (2x) EVGA GTX 570 2.5GB 025-P3-1579-AR $320 EVGA GTX 560 Ti 3 (1x) EVGA GTX 670 FTW 2.0GB 02G-P4-2678-KR $420 EVGA GTX 560 Ti
Option 1 will probably hold me out for the time being. I can run the games I (currently) play on max without a hitch, so is probably the most attractive option.
Option 2 would be ideal, but for the disadvantages previously listed. Option 3, while pricey for at times limited improvement, would ensure that I wouldn't see terrible frame rates for a longer period of time.
With Option 3, I can probably sell the GTX 570 to a friend for a good $100 - $150 less than what it's selling for new, dropping the price of the GTX 670 to an ideal ~$300 (disregarding the $380 I spent on the GTX 570 in the first place).
All Options retain the 560 Ti for Physx, which is overkill, but the card has nothing better to do at the moment and outperforms the 9800 GT.