AMD does a lot a lot of fuzz around the 64 bits version of the game Far Cry.
Anadtech tested it. Though the increase in performance is not that obvious, the optimization regarding graphics and texture is more impressing. Here are two screenshots from their article. The first comes from the 32 bits version, and the second from a 64 bits system.

You can feel the difference.
The point is, this improvement has little interest in that respect that 64bits Windows XP has been officially available for a few weeks only.
It would have been smarter to do that on a Mac system, which has been providing both 64 bits processors and operating system for a bit longer...