as you might have already noticed i'm not the biggest fan of VISTA at all ...its using too much RAM and comes with too many bells and whistles for my taste ...
as usually i'd guess like everything from MS gets usable with a *3* in it's name (windows 3.11, windows 2000 SP3, ect) ... ok, its getting better, XP SP2 is ok ...
my strategy would be the following: have 2 disks, partition both into 32-48 GB primary partition and the rest, install XP64 into the first partition of the second harddisk (boot loader remains on disk 1), install VISTA64 into first partition of disk 1.
now you have a dual boot system and whenever you decide you could wipe XP64 without having to change anything but deleting one entry from the boot loader..
AFAIK you are always allowed to downgrade one version with MS operating systems, so you would actually be covered with just a VISTA64 license (because you can't run both simultaneously)
christian