No problem with your high-end system and a pair of GTX 650 Ti Boosts. In fact, it's a really good idea due to the fact that you can get top-level performance for just $340. Don't fear SLI, Nvidia does it really well and has excellent driver support for new games.
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"After running the GeForce GTX 650 Ti Boost SLI through our test suite, I have to admit that I'm impressed. The duo delivered performance easily matching and often exceeding much more expensive single-card options such as the GeForce GTX 680 and Radeon HD 7970 GHz Edition, and they don't cost as much. SLI multi-GPU scaling works well with all of our titles except for F1 2012. Scaling by going from one to two GTX 650 Ti Boost cards is around 70%, even with F1 2012...