I agree that SLI is most useful when you've had a single high end card for a bit and then want to do the cheapest upgrade possible by putting another into SLI.
2 x 560ti 2GB is a pretty nice setup ... BUT at this point, starting w/nothing? I'd go with a higher-end single card, which is a lot more 'upgradable' down the line.
The 7950 is a pretty sweet choice at the moment, but I'd consider waiting for Kepler to make any big moves.
If you just need something 'for now' I'd look for a cheap used card like a last-gen nV or AMD card (4xx or 6xxx series).
However, I take issue with the 'SLI is a pain' statement. It's actually pretty dang reliable and works well. Sometimes new games come out and it takes a bit for nV to get a SLI profile out for the game, but typically there's workarounds possible using nV Inspector if you just can't wait.
It is important though to have a board that's set up well for SLI with proper spacing between the slots so the cards aren't sandwiched too close together