I don't think Win7 is any faster for gaming then vista is. I know I saw articles when Vista first came out showing XP was faster for gaming, but that changed some time later. I haven't seen any articles showing which OS is best.
The only gaming centric difference that Win7 has over Vista is the ability to load more then one graphics driver. Vista will only load one, while XP or Win7 doesn't care. The only reason a gamer would care is if he uses an AMD video card and wants to run an Nvidia card for PhysX. If this isn't you, then you wouldn't care.