All games are compatible with both AMD and Nvidia.
However, certain graphical effects can only be enabled on Nvidia cards. Mostly PhysX, also rarely HBAO+ and Hairworks. TXAA as well, but TXAA is garbage so that one's irrelevant. Really it's mostly PhysX that you'd have to worry about in mainstream games.
The i5-4670 is a very strong CPU, so ofc any graphics card is going to hit its limit before that CPU.
However, the R7 260X is a good choice, and a better value than the GTX 650 Ti for most purposes. However, PhysX is useful and will make the GTX 650 Ti the better choice for anyone whose favorite games include PhysX titles.
As it'll likely be a straight port from the Xbox version, I doubt GTA V will use PhysX. Hence I'd recommend the...