Nobody knows for sure how demanding next gen games will be, but by most guesses it should run the games at medium, theres no guarantee how much fps you are going to get. It could be 30 or 60 or 5. Nobody knows for sure yet.
For gta V theses are the released specs:
Minimum Grand Theft Auto V requirements:
Processor: Intel Core 2 Duo 2.6 Ghz or equivalent
Memory/RAM: 2 GB Ram
Video Card: 512MB NVIDIA 7900 or 512MB ATI X1900
Hard Disk Space: 30 – 40 GB
Operating System: Windows XP SP3 or Windows Vista SP1 or Windows 7 SP1
Recommended system requirements:
CPU: Intel Core 2 Quad 3.66GHz or equivalent AMD
RAM: 4 GB
Video Card (GPU): NVIDIA GTX 480 or similar ATI
Hard Disk Space: 40 GB free HDD
Operating System: Vista SP 2 or Window 7 – 8 SP 1
So these are all estimated, but close at that, so for gta v you are in the clear, watchdogs will be a closer cut though as those are minimum requirements. When I was playing Watchdogs at E3 they had some serious hardware to be running it on high.
As for any other games im not too sure on it.