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.
"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 taken into account. Unlike AMD, NVIDIA does a good job of maintaining its SLI profiles, so you should be able to play new games without a long wait for multi-GPU support. However, the risk that a game will not be supported still exists, and you might, at worst, end up with single-card performance. This is in my opinion, given the massive performance-per-dollar advantage, an acceptable tradeoff. I would definitely recommend a GTX 650 Ti Boost SLI setup to a friend looking to spend as little money as possible on a high-end gaming rig."http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_650_Ti_Boost_SLI/23.html