The X-64 bit seems (IMO) to run a bit snappier if you have => 4GB RAM. (Some people also report it works fine with as little as 2gb ram but I'm not sure what the advantage is in using a 64 bit OS on < 4GB RAM.
The main disadvantages are -- some TV cards won't yet work with W7 X-64 (Hauppage and Pinnacle) although you can use Windows Media Center to get some functionality out of these TV cards -- a big drawback is that the Windows Media center 1) doesn't allow switching between Antenna input say and Composite Video )
2) If you initially set it up for Composite Video (say you want to connect the output from a Video Sender or Satellite box it will insist you have an IR device as well even though the set up will capture the Composite video.
On W7 X-32 these TV cards generally seem to work using the VISTA 32 bit software -- so if TV applications are important I'd just hold off on the 64 Bit version for a while yet -- by all means test other apps however.
You can Triple boot without a problem -- best method is to use some sort of Boot Manager. When you install choose a different partition to where you've installed your other OS'es (naturally).
Just google on double boot, bootmanagers etc etc. There are many ways to do this -- you've got the base correct -- if you want to still boot XP that has to be the 1ST OS installed which is what you have.
Cheers
jimbo