64-bit XP or 64-bit Win7 has an advantage here, because you can have more
than 4GB of RAM, and allocate a few GB to each virtual machine running
simultaneously.
4 GiB of RAM was the limit before Pentium II, but after that, the
internal addressing capability was 36 bits, with potentially 64 GiB of
physical memory addressability. However, some of the earlier
processors did not bring out all of the extra high order address
lines, limiting the available physical memory somewhere between 4 and
64 GiB.
IMHO, the only reason for using processors with 64 bit virtual address
space is large data bases. It is very convenient to map a disk based
multiple terabyte databases (spanning several physical disk drives)
into a single virtual address space and access the bytes on disk by
simply making memory references. The OS page faulting mechanism will
load the referenced pages into memory and also write back to disk, if
RAM is needed for more urgent needs (or by explicit flush requests).
Thus in practice, a few gigabytes of actual RAM will serve as an L3 or
L4 cache, while the real "memory" is spread over several disk
platters.
Of course 64 bit virtual address spaces are not a new thing, I have
used it on DEC Alpha since mid 1990's and I guess there have been
earlier implementations.