When are you planning on upgrading? Also the new AMD cards are launching this month so id look and see what the performance and price of those are gonna be. But id say get a 770 or 7970ghz if you cant wait
First, I assume this is for playing GAMES? (In which case you'd likely play on one monitor only)
1) The CPU you have is also important to prevent bottlenecks to the graphics card. If you have an i7-860/920 or i5-750 (and comparable AMD) or better you should be okay for most games with the GTX770.
2) It also depends which games you play. If you plan to play demanding games like BF4, Metro, Cryis 3, Far Cry 3 etc you want the GTX770 for sure. Many new and older games will still run awesome on the GTX760.
GTX770 - Asus DC2OC
GTX760 - MSI TF
AMD new cards.
Not a lot of solid info yet but it might be worth waiting. I believe you'd be looking at one of THESE cards: