PAE is enabled by default on most systems anyway, to enable DEP.
I know on regular XP, using PAE does not enable more than 4GB of memory, no matter what. Server 2003, however, could see all my 4GB of memory when I enabled this.
There's an interesting article on this somewhere, which points out that PAE is not the magic solution to break the 32 bit barrier, but I can't remember where it was.