Quoting the official release, "With an AMD64 processor and the new Windows XP Pro x64 operating system, you can now enjoy Far Cry with additional content, higher frame rates, enhanced textures, longer view distances, and more objects with real-time physics."
Bloody lovely stuff then. And all for free. The in-depth examples on this site will give you more to chew on.
We've included the screens here for your perusal, including some comparisons between the 32 bit and 64 bit versions, and a new multiplayer map.