The service pack didn't change how much is supported just how it reports what is in the PC.
I'm not 100% sure but i think it went something like this.
XP told you what was installed in your PC.
Vista told you what was addressable in your PC or to put it another way how much Ram that was actually usable to you.
People didn't like this and it caused lots of confusion so they changed it back with the service pack.
At least that's how I remember reading it on various net forums.
Mactronix