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
Build 7022 was never ment to be released.. it's just a daily build... that was leaked.. Build 7000 64 bit works VERY well though so I'd say give it a try.. 
