I'm torn between these 2 cards too. I've been reading around a lot and some say that 2gb are becoming too little for modern games, especially for BF4 which supposedly easily saturates the 2gb video ram when on ultra settings and filters set to max @ 1080p. Now I know the 770 may not be capable of processing 4gb of video data, but it's not that you're gonna use all of the available ram, is it? The 2 extra gigabytes give us some headroom for higher filters. So even it fills up to, say, 3gb of ram I guess the 770 is going to handle it pretty well since it performs better than the 280x.
The question is if the 770 is worth the premium over the 280x. In Italy you can get a Sapphire 280x for 270 euros while a 4gb 770 from gigabyte for 320.
I'm starting to think it may not be worth it, since a 770 is something like a few fps faster than a 280x. 50 euros more is like 5 to 10 euros a frame, not so cheap