I'm talking about the "new Mac Pro". The existing one a better piece of kit if you ask me.
You can change the CPUs officially with the HP. I doubt Apple will offer that option.
My current "workstation" is configured with 2x 4 core Xeons, 32Gb of RAM, 2x Samsung 840's, a 5 drive SAS array, a 32Gb PCIe RAMdisk and 2 Quadro cards. That won't fit in the new Mac Pro...
I also don't see how the machine you linked to supports "chuck another CPU in". It looks like it limits you to two, which is the same as the Mac Pro.