XP definitely needs less memory, however it goes out of support shortly, so you won't be able to get any security patches.
32 vs 64 bit makes no significant difference in the memory footprint or performance.
Upgrading to 4GB of RAM should let you run Windows 7 OK, but only on 10.6 or 10.7, and without much else running on the host. With 10.8, you really want at least 8GB in the machine for decent performance. Keep in mind that 10.6 is no longer supported by Apple.