Here are the head-to-head benchmarks http://www.anandtech.com/bench/product/1041?vs=1037
Looks like the 280x and the 770 are very comparable cards.
What resolution do you play at? You won't need 4Gb if you're playing at 1920 x 1080.
Which would sli/cfx better? Nvidia has historically been better at delivering drivers and resolving issues. I can't tell you if AMD has caught up or surpassed though.