The x64 edition is virtually identical to XP Pro in terms of features. The only major difference is that the x64 version is made for CPU's that are 64-Bit. Right now it's pretty pointless to get it, since there are still major compatibility issues, mainly with drivers.
I would wait until Vista before buying a 64-Bit OS.
They are both basiclaly the same thing. Except the the 64 bit edition is made for 64 bit cpus. I would not get the 64 bit edition cause there arent many drivers avaibale for it and some of the drivers can be incompatible. I would wait for Vista to come out and it could be a long wait for it to come but that will be a 64 bit OS and should be alot better.