SP3 has a tweak to placate the owners of 4GB machines. It doesn't change how Windows works in regard to the RAM or if you want to try the /3GB switch but it stops a few thousand support calls a day.
I currently have Vista Ultimate 64 bit installed on a Macbook Pro but I'm considering switching to a 32 bit version of either Vista or XP because of some software licensing issues. The problem is I have 4 GB of RAM installed and from prior experience with my desktop (Dell XPS 720) I've learned that 32 bit OS's can only use a portion of that RAM. Before I switched to a 64 bit OS on my Dell it was only showing 2.75 GB of RAM installed. I was using Vista Home Premium at the time. If I installed XP sp3 would I experience the same problem?