GTA5's port to PC isn't soley for PC systems. It's also for next generation consoles.
This is significant because Rockstar has re-released games in the past for next generation consoles, and they usually do something interesting with it. A good example is when they ported Bully to the 360 and it turned out to run on a completely different engine with a much higher polycount.
With GTA5's next gen port a lot of people have noticed Rockstar has changed the map around and added more vegetation. It is possible they've begun adding more content specifically for the next gen release.
As for 64 bit system support. GTA5 was a very ambitious game and it is likely it will require a very powerful computer to run.