I'll be putting a lot of applications on my OS drive -- Adobe CS6 Master Collection among many others so it'll fill up over time, and I'll be leaving 25% free at all times. Maybe I'll keep some of my documents there too.
I already bought the drive -- a Samsung 840 Pro -- so I wouldn't want to use that just for games.
Ah ok well from your first post it looked like you only intended it to be used for the OS.
A 64gb drive for games, well for me anyway wouldn't be near enough. Some of the games I have use over 20gb for a single game, that being said how many games do you have/plan to have on your computer? I would get a second 256gb SSD for games or at the very least a 120GB one... Anything less than that, it might not be worth it, unless you don't mind moving games around a lot to get the one you currently are playing on to the SSD and moving others to the HDD to make room for it.
I don't play that many games simultaneously. I'll usually play one shooter, like Fallout: New Vegas or Bioshock 2 over the course of a month and then move on to another.
I'm not an avid gamer... but when I do play I like chapters to load quickly. To give you an idea of how old school I am... I'm patiently waiting for Doom 4, Quake 5, Fallout 4, Bioshock Infinite... and the ultimate pipe dream, Half-Life 3!