i can't imagine that wider adresses make much difference in ram usage. if you think of a typical program and how little space pointers take there compared to the data... but maybe i am wrong...

i think installing two windows versions isn't worth the effort. i currently use 64bit XP (with only 4gb of ram) and don't have any problems.

if you need an older windows around for some reason then virtualization (for example with virtualbox or vmware) could be nice.