It's not certain if there's going to be different DCPUs with different specs. Notch has said[1] that there's definitely going to be versions with different clock rates floating around in space or on abandoned spaceships.
Notch has said that when he "upgrades" the DCPU spec, the old CPUs will continue to work as they have, but new CPUs will become available. I don't have the reference handy though.
[1] http://vps.thomascomputerindustries.com/logs/freenode/0x10c-... (search for "[11:38:45]")