It's also interesting to note that 32 bit XP does in fact have 4GB of address space, but there's lots of other things competing for that space. What's competing for that address space? Things like System ROM (BIOS), APIC(s), Integrated PCI devices, such as network adapters and SCSI controllers, PCI expansion cards, Graphics cards, PCI Express cards (if applicable). You can certainly install 4GB of RAM under Windows XP (as I have for machines I've built for friends), but your operating system and applications won't be able to access more than ~ 3.4 GB of it.
XP Pro on all four machines here....and no plans to move away from it.
Regards - B.B.S.
. I'm keeping XP pro until DX10 improves and video card drivers get a little more backwards compatible in vista.