We'll I'd disagree a bit, I'm running two (2) R9 280X Sapphire Vapor-X cards and have no complains on CF. Plays ANY game max'd out @ 1920x1080.
But, there's no way to make it go faster, when it's need.......
So, I always start off a build with fastest single card I can get/ (ah, afford). So, at this time, yeah GTX 780Ti....... as it allows the most potential room for future upgrade without trashing that original card.