What benefit does SLI/crossfire have on a single screen set up?
There are a few different benefits, for gaming it would be frame rates. Let's break this down into a few categories: 2560x1600, 1920x1080 and low resolutions (1280x720, 1600x900, etc). Then we have 120Hz screens and 60Hz screens.
The 120Hz/60Hz thing is all about frame rates - trying to keep your frame rate locked to the refresh rate of your monitor. So on a normal 60Hz screen, your image in-game will look best at above 60 frames per second, and 120Hz obviously translates into 120 frames per second.
I could go on forever in those frame rate discussions, but let's say you're running a 1920x1080 display. A multi-GPU setup like Radeon HD 7970's in CrossFire would provide much more than 60fps in every game on the market right now at maximum detail. Then we'll need to talk anti-aliasing. Anti-aliasing will really tax frame rates, which is where bandwidth on-GPU and copious amounts of VRAM come into play.
So, would a multi-GPU setup help for single screen gaming? Yes and no. You will definitely see huge benchmark scores, but for actual, in-game results? It depends on the settings in-game and of course, the game itself.
I recommend multi-GPU for single screen gaming as it will provide you with over 60fps which is what you want for perfect gaming conditions. It'll also give you some huge GPU headroom to enable some, if not all anti-aliasing options in-game and not dip below 60fps.
If we're talking mid-range GPUs like the GeForce GTX 660 Ti cards, you will see 60fps in most games - but the AA will be something trickier.
As always, if you're looking at SLI or CrossFiring up two GPUs - don't go mid-range. Get one fast card now (HD 7970/GTX 670 or 680) and then SLI/CF in the future. That's the best way of doing things and gives you some incredible future-proofing.
If you want to talk in detail about this, and give me some more details on your system - feel free to e-mail me and I'll walk you through it.