If you're getting good performance right now, I'd be tempted to hold off until the 28nm cards arrive. Should be a pretty big jump in performance this time around.
But if you want to upgrade now... well, in terms of raw performance, 460SLI vs single 580 is pretty similar [see here], but you will get smoother gameplay with a single 580. It's a more expensive option though, so it's hard to say if it's worth it. If it were me I would just wait for the next gen - if you're going to watercool a card you want it to have a decent shelf-life.
This compartment shows very similar results.
When is nvidia going to release the 28nm chips?