Well, what CPU do you have? Also what PSU is in your system? We need to know these things or you could bottleneck a new Video card.
GTA5 isn't even out, and no system requirements have been announced. And even then, GTA5 could fall in the same foot steps as GTA4 did being poorly optimized. It far to soon to tell.
Assassin's Creed unity is so broken and badly optimized its not even funny. So to max everything out with todays game at 1080p, I recommend a GTX780ti or a GTX980. AMD has the R9 290x that will do well, but I don't recommend it for temperature reasons. For AMd I'd wait untill they come out with their newer series, though the Omega drivers I herd are doing really well.