3 Ram to run GTA V?
You need like 4 RAM or more and a Powerful graphic card to run that game at minimun graphics.

Otherwise it will lag as hell, anyway try to install it and see what happens because i am not expert at game perfomance.
Also the Win 7 Ultimate 64-bit is the best choice IMO!
Oh, since my dad, that knows a lot about PC, thanks to him i can run GTA V without framerate drops, the PC doesn't scream, i mean, it's doesn't make noise and it keeps cold, i don't need overclock because it runs good and also with overclock the graphic card damages, i mean, it has less time of life.
P.S: I don't play GTA V but i have seen it running in my Gaming PC.