Games Optimization - What do you think?

I don't know if I wrote the thread name right.. excuse me if my english is annoying you ..   So, here is my problem:
Right now I am running an Windows XP Professional SP3 32bit. I am taking now a 4GB RAM(my pc is old..), so, I am going to.. upgrade.. to 64bit. But I am sort of.. confused..?
I am going to use my PC for gaming, only for it, and I don't know which OS to take. Win7 Ultimate 64bit SP2, or WinXP 64bit SP2? I would prefer the XP, cuz I am now with it now, and I've never used an Win7 OS. In the other hand, Win7 is.. younger(newer) than the XP...
Which OS should I choose? I am questioned 'cuz I've heard that some games are not optimized for Win7, or XP. I am going to play games like.. Assassins Creed 1,2,3, Revelations, Brotherhood, COD MW2,3, COD Black Ops, Battlefield Bad Company 2,3, Batman AA, FIFA 11, Fable 3, Wither 2, MassEffect2, Infamous..some of the games are not announced yet, but I guess u got it, I am thinking od using the OS for a long time. I just don't know what to choose, which one is better for gaming, and I won't have problems with any kind of game for.. for a long time, lets say.

And I would like to say that I would appreciate it, if the people who answer, are already used the both Operating systems.

Cheers