It depends, a single GTX 660 Ti can be configured with 2 or 3GB of VRAM, whereas a GTX 650 Ti Boost can only be found with 1 or 2GB. A single GTX 660 Ti would consume less power than dual GTX 650 Ti Boosts in SLI and would also be quieter than a Dual GPU setup. Dual 650 Ti Boosts perform a bit above a GTX 670 but factor in what I just said before. Overall, dual 650 Ti boosts are slightly more expensive than a Single 660 Ti but in my opinion I would go with a GTX 660 Ti as you have the option in the future to add a second card and increase your performance much more significantly than a dual lower tier GPU SLI configuration... :)