XP will actually use 3.5 GB or RAM. So will Vista - unless you get the 64 bit version! That brings many other problems.
We once spent all evening getting a colleagues Vista64 setup to work with all the required company software (designed for XP 32 bit). After getting various weird 64bit versions of components and doing required work-arounds, we had it! Then came the bummer - the VPN client was not supported on Vista64! All that work down the tubes for nothing!