“What Windows XP Pro x64 Edition is bringing to the table for the gamer is OS support for 64-bit applications and games that can take advantage of 64-bit CPUs like the AMD Athlon 64 and Intel’s EM64T technology. Windows XP Professional x64 Edition is unique in that it will also allow 32-bit applications to run on the OS alongside 64-bit applications. This means the games and applications you use now should work with this OS. You do need 64-bit drivers for your hardware components, and we suggest downloading them before installation.”
Check out the full comparison at Hard|OCP.