Video cards in gaming don't fully scale or double for each card. 2 cards don't usually give 2x the performance in games. In GPU processing, then they do as it's processing but for gaming, they don't. Usually it can be anywhere from 0% increase from a crappy coded game to 80% for a well coded game and drivers but usually a 50% increase. I don't know exact benchmarks to see if 2x660 is faster or not against a 670, but figure on average 2x660's is really about 1.5x the 660's power. Running two cards might also mean a new power supply as two cards get greedy on power, good sized case and fans for cooling as the extra card right next to the first card creates a lot more heat. You need a good SLI board and not a crappy one that limits the 2nd slot to a 1x slot or anything.