The Gtx 780 is better at high res then the Gtx 770, because of the chip. Memory matters indeed but it matters more if the card is fast and has enough cores.
Do you want the Gtx 780 because you get a free copy of watchdogs? Because on 1444P the R9 290 is a bit faster then the Gtx 780 and also a bit cheaper. Though it runs hotter, and there are a bit more people that have problems with the card. Though I think it's woth to consider because it can save you some money.
Another option is wait for the Gtx 880 release, because the prices of other graphics cards may drop too.
But if you think you really need a upgrade now then upgrade it!