If you look at the bridges on the chip itself, where the L5's are you'll see something like this;
Early XP's; I::I
Late XP's; I:::
What you need to do, on the later build XP's, is to connect that last set of bridges, so they look like the first example above. That's it. If you've got an early XP, it's already connected (or, actually, AMD didn't cut them).