My 2nd 660Ti arrived yesterday, and got it SLI'd with my old one. Damn nice improvement. The only game I have installed with a built-in benchmark is currently the new Tomb Raider, and it went from avg 50.1 FPS to 95.7 FPS with the second card. The max and min FPS numbers are odd though. The min FPS went from 28 to 40(not a massive jump) but the max FPS went from 68 to 145(way more than double).
Who knows, I'm mainly just concerned with the average, and that the min doesn't drop below 30 in most games, so that was a nice upgrade regardless.
I ended up using 2 different brands of cards, my original was an EVGA card, and this one is a Gigabyte card. The only downside was the the EVGA was more overclockable, I constantly ran it with a roughly 10-15% overclock (GPU + 70mhz and memory + 600 mhz), but the new Gigabyte won't hardly overclock at all, so had to dial them both back to stock speeds for stability. Oh well, still a damn nice upgrade for ~$180(I have the 3GB cards)