Not sure why you tossed out $400. I would think you'd be looking at a GTX 970 for $300. I personally stay away from SLI for two reasons:
1: you get higher average FPS, but your minimum FPS is often substantially lower than a similar performing single card solution
2: support for SLI is generally pretty good these days, but depending on your interests you may still find games that are either sloppily released or in some sort of perpetual beta state that just never seem to get around to adding support for it fast enough — in which case your great SLI frame rate will be nowhere to be seen.
3: If you pay your own power bills, using a single, powerful card rather than SLI can cut your total system power draw by anywhere from 100-250 watts, depending on the hardware options. That's actually quite a lot of electricity. For someone like myself who has a 3d environment up on my screen for a major part of the day, that kind of electricity represents some serious bucks in the long run.
This is how I count to two.