I used to run SLI GTX 570s, I can definitely tell you that you should try to get the most powerful single GPU that can sustain the monitor FPS in your resolution/game of choice. I know most people can testify that there are virtually no difference between SLI and single GPU smoothness but I'll give you my experience for what its worth.
I got the GTX 570 a short time before the AMD 7970 was released because I was poor, good card but not very future proof and didn't have quite enough oompf I needed (my primary gaming was BF3 on 1080p), I regretted not getting the 3gb GTX 580 at the time. The 7970 was astronomical in pricing so a single GPU upgrade was out of the question. I got a second 570.
Basically, with dual GTX 570, I had a massive performance buffer in most games (especially BF3) because I capped the FPS to 59 and would never get slowdowns below this. This was great for BF3 and the games of that time because of this buffer. However as time progresses, the SLI rig was less and less capable of exceeding that 59FPS cap so what I noticed was what you can describe as microstutter, while its not actually stutter, its more of a lack of fluidity in the presentation of the frames.
The irony is that the harder hitting games/settings are the ones that require SLI performance but conversely the smoothness is the first to be compromised. Basically, SLI is great for ensuring performance never drops below 59 FPS by giving you a buffer but if you need SLI to achieve 59 FPS then smoothness suffers.
So whats my point? unless you really want to ensure you never get FPS drops at a given resolution/game/setting don't SLI, use that money and put it towards a future single GPU. Towards the end, when my SLI rig was only producing 60FPS in most games at the ultra settings, I found myself disabling the second card and simply lowering the game settings for the sake of smoothness.
In hindsight, if I had pocketed the $370 I spent on the second 570, I could've paid more than 60% of the 290 I am using now which is fully 30% faster than BOTH 570s.