You really only need to upgrade to XP64 if you're going to use more than 4 Gb of Ram.
Also, make sure that the software that you use can handle a 64bit OS, and that all of the devices that you use (scanner, printer, midi cards etc) have working drivers for XP64.
I'm in the same boat right now. I want to have a Quad-Core with 8 GB RAM and have to make sure the needed audio hardware works with XP64. Unfortunately any info about the drivers seems to be very rare.
I have to different OS partitions on my system, Windows XP Pro (x86 version) and Windows XP x64. I pick which OS I want to use depending on the plugins I need to use.
Windows x64 has been working great for orchestral work. The Kontakt 2 Player is a little goofy inside of Sonar 6 x64 version as it has to go through Bitbridge. You need to move the Kontakt screen over when deleting or replacing instruments so any prompts to save aren't covered up. They appear behind the Kontakt 2 Player window which then can't be closed so there is no way to answer the save window. The new ARIA player and the VSL VE player are both x64 so they work great. BTW, I haven't had one crash in x64 which require a reboot.