The D510 is an x86-64 processor. It can run 64 bit and 32 bit operating systems. It's the same instruction set as the core 2, i7, and everything in between. It's as real as any other '64 bit' processor you'd buy today.
Physical address extension (PAE) means that the processor will also allow a 32 bit operating system to access more than 4GB of ram, if the OS supports PAE.
All of this only matters if you have a machine with 4GB or more of ram, and want to install a 32 bit OS for some reason. Like maybe you already own a copy of XP 32 bit, or you use linux, and things break when you use the 64 bit version. I have an Atom 330 with 4GB or ram, and use 32 bit OSs.
I hope that clears things up.
do you have to manually enable it?
I manually enabled my workstation for 4gb, windows 7,noticed nothing at all.. safe to assume common sense finds that it is 4gb and the os deals with it accordingly? or does something need to be done?