still I reckon 1gb vram would be a limiting factor for future gaming. Newer games are already starting to use more than 1gb vram and the amount would only continue to grow. Gtx560 sli would only be good for another few months or so and after that, op will have to upgrade. So why not upgrade now and get a relatively higher resell value for the gtx560 ti(before the release of the gtx660 series).
A single gpu solution is always better than sli imo.