Right now I've got two GTX 580's in SLI (gigabyte soc and msi twin frozer II, both 1,5gb versions). I got two monitors connected to them and a TV for couch gaming/XBMC.
I've been looking around a bit to replace them with a single new card, a GTX 780 would set me back around € 500 (I should be able to get around € 150 euro per card if i sell them online though, maybe more if I'm patient). But if I buy a GTX 780 I wouldn't be able to use all 3 screens at once, so I'm looking at the AMD equivalent as well.
Anyway, how much of a performance increase would I get if I buy a current generation single card and would that increase be worth it to upgrade?