The Ultimate disk, if retail, should come with x64 in addition to x86.
If you have OEM, you're going to have to buy an x64 disk.
It really depends what you're working with. If you're gonna run 64-bit games and productivity software, then go for x64. If you're gonna run your same old 32-bit software, you will notice a performance decrease. At least i did when i used to use XP x64. I noticed slower Super Pi scores, 3D Mark scores and about 10FPS loss average.
So if you're gonna use 64-bit software, sure, go ahead and use x64. But if you're using 32-bit software, no reason to sacrifice compatibility and performance just to say, "Aha! I've got a 64-bit OS!" Might as well wait for x64 to become mainstream if that's the case.