Well, i have the Athon 64 4200 which is 64-bit and i installed XP Pro 64-bit edition on it, and didn't notice any speed differences. THis is probably because most software is only now starting to release stuff that utilizes the 64-bits, whereas most of the stuff is 32-bit, which makes the 64 useless really. But as the previous poster said, now that Vista is coming out, i guess things will use 64-bit more, so in coming months, yes there will be a good difference.
Matt