Since you will be playing multiple games, here are my 2 cents.
SLI scaling depends greatly on the game. Some games take great advantage of SLI/CF, others see only marginal or no gain in performance. To accommodate your needs as a gamer, I would, in my humble opinion, suggest staying with a single card configuration.
The GTX 670 you mentioned is 3 tiers higher than the 560Ti according to Tom's GPU hierarchy chart, which means you will certainly see a notable performance gain.
I would like to ask you for your system's current specs, as the 670 is an expensive piece of technology and may go under-utilized depending on the rest of your system.
i5 2500k @ 4.5 ghz
galaxy gtx 560 ti 448
asrock p67 gen 3
XCLIO STABLEPOWER GOLD 1000W Full Modular
LIAN LI Lancool PC-K62
corsair 8gb 1333
hyper 212 heatsink