A single more powerful card is almost always better than a multi card setup. Not all games support Sli or Crossfire and there is micro stuttering to deal with although Nvidia cards have less of an issue there than AMD cards.
As ASHISH65 said the Nvidia 7xx series will be released at the end of May or early June. It will just be a refresh of the 6xx series but should give a decent performance boost. It would be worth waiting to see how the 7xx series performs and affects prices before upgrading in my opinion.