I am currently putting together a new build, using i7 (for CS6), and decent nVidia (CUDA) GPU, for photo and gaming. Unless I cut back elsewhere, that I don't really want to do, the best GPU, I can get, is GTX660Ti. That will be fine for my current res of 1920x1080.
In the near future, however, I hope to upgrade monitor to 1440, or 1600, so I'm thinking, if I get one GTX660Ti now, I could add a 2nd, in SLi if, and when, I do upgrade monitor. Are there any "downsides" to running one monitor, off two graphics cards, in SLi?
No downsides to running two graphics cards on one display, however you have to remember about GPU scaling, which can affect your results. However the 660ti cut down memory bandwidth will severely affect it in games with AA or higher resolutions. There is a T.H article about it somewhere, I will find it for you.
This is a good way to show how it can affect frames, you can see the $250 7870 beating the $400 670 in what I believe is a Nvidia preferred game.