Many of the x64 equipped BlenderHeads running on Linux (let's face the truth, Windows XP 64 exists only on MS's products list, it isn't real, it's a state of mind) wondered at least one time which are the advantages in using a 64 bit version of Blender over a 32 bit one; time has come to unfold the truth.
Recently a real usability issue for 64 bit version of Blender was pointed out on the Bf-committers list: actually a file created on such a version cannot be opened at all on 32 bit machines; this goes ways ahead from the old library-append incompatibility meaning that some real work is needed to have 64 bit versions ready for some real production works.
"For now just be extremely cautious with the 64 bits version" - Ton commented - "there's actually no real reason to use a 64 bits build, unless you use projects that demand a lot of memory (>2 GB).
Blender also stores a lot of pointers in its data structures, which means that memory consumption for editing and rendering will increase in 64 bits with like 25-30%. So if your system has less than 2 GB of memory you'll be better off using 32 bits.
For tests it is interesting to check on speed benefits though!"
So, ladies and gentlemen, you're aware of the problem now, simply think about pros and cons before going the 64 bit way.