So I was originally going to get a 780 lightning but then my friend offered to sell me his gtx 690 for $500 (upgrading to a 780 ti). I game on a 2560x1440 monitor. I would have loved to get an r9 290 but it looks like the after market cards aren't coming until January.
I've never had a multi-gpu configuration before. Essentially:
2gb with 256 bit bus vs 3gb with 384 bit bus for 1440p
SLi vs single gpu frame times
600 series technologies vs 700 series (i.e. gpu boost 2.0)
$500 vs ~$550