Windows 7 is still supported, XP support is dropped. 64bit is required.
GPU won't really need 2GB, depends heavily on the game, most games are fine with less than 1Gig.
Sorry, I couldn't remember if it was windows 7 or xp. As Simon said, you probably won't need exactly what I said, but it's subjective & each game is different, so you can't just say all games made with VS require x specs to run, which is also true for all other game engines.
As for your issue of not having windows, I would recommend considering installing a VM on your machine & installing windows on it, which version is up to you. Or you could look into dual booting. Or you could borrow a windows laptop off a friend or family member. etc.
In regards to VM (virtual machine), how well windows would run would depend on how powerful your machine is & which version of windows you install as technically the machine would still be running your original OS in the background, so don't expect the results/speed to be as fast/smooth as running an actual windows machine.