I concur with the 780 ti sli, at normal prices I would say 290x, but most people aren't lucky enough to get them at normal prices. Then there is the heat, I use mine to mine at night and help heat my room (no joke).
Edit: Just read price doesn't matter get the 780 I have had driver issues with my card on 13.12.
Use zotac gtx 780 amp edition for sli. This is the most powerful gtx 780 variant right now. Its 12% more powerful than a stock 780 and only 2-3% less than a stock 780ti but costs pretty less. Also this card is a very very good overclocker while remaining cool and silent. It requires less power than 780ti and much less than r9 290x . So don't think twice just go for it!!!! Happy gamin
AMD cards and software are garbage. If you want a polished/refined product with fantastic support/drivers/software and without question superior built hardware, go Nvidia(And make sure you back it up with a quality Intel Processor too like a 4770k etc). As proven, raw FPS does not alone dictate quality/performance. Too many other factors involved to ignore.