The best/fastest solution might be a $1000 GTX690.
The GTX680 sli under the hood can be very fast.
But, I would try a much cheaper GTX680 with a factory overclock.
I think you would find it very effective.
I have two such 2560 x 1600 monitors, and have no problem with a stock GTX680 gaming on one.
I get a steady 60fps on civ 4, but I can't say how it would do on different games.
If budget is not an issue, then you could always add a second one later.
And... Do not pay extra for a 4gb version, read this:http://www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154