So much I know, Vista will run with more than 3.6 GB RAM.
If you have all the driver you need, this could be a solution.
It will also run with 64 bits, no matter what application.
XP 64 bit may ba also problematic, if the software if not
perfectly adapted to a 64 bit system.

I use now XP 32, but some of my programs exploit the benefits
of the dual 64 bit processor I have. For music it´s quite OK.
BTW: It makes no sense to overload an old computer with RAM.
Newer, faster RAM chips, faster busses and SATA make the
new PCs faster. An AMD dual or quad CPU is cheap today.
I am satisfied with the one I have.

Regards,
Djordje
www.midiguitar.de