Is it the same in Vista?
A new computer at work came with Vista. I could swap my XP with the Vista if it would allow me to use more RAM?
XP and Vista have the same effective memory limits - if you want more than 4GB of memory you'll want to use a 64-bit OS (XP, Vista and Win 7 all come in 64 bit flavors). And note, even with 4GB, you'll largely be limited (out of the box) to 2GB per application.
Seriously, though, if you're going to upgrade from XP go to Windows 7, not Vista. And Win 7 64bit is perfectly fine - unless you're running legacy peripherals that have no 64 bit drivers you'll be much better off on Windows 7 in 64bit mode.
Also, note that these are simplifications - 32bit Windows (Whether XP, Vista or 7) has so many different ways of tweaking memory config that it is possible to use more than 4GB, or more than 2GB per app, using various tweaks like PAE; but it is *far* easier to just run the 64 bit flavor and not have to think about this, assuming you don't have problems that prevent you from running a 64 bit OS.