Sorry to reply to my own post, but was just re-reading the anadtech review and they mention that the Ultra is in fact exactly the same as the 6800Go, except for the core/mem speeds it runs. It mentions that the 300/300 clocks seen from most 6800Gos are due to the fact that they are in P4 machines, whereas this ultra is with a PM. I can't find any more info about the NV42m except that a guy at Beyond3d claimed Dell told him it was the core in the Ultra.
This is the direct quote from anandtech that makes me suspicious: "Rather than just ship their Go 6800 at the high core speed, NVIDIA has given it the Ultra moniker to differentiate the product."
This would technically mean that providing the 6800Go comes with GDDR3, it will be just as overclockable as the ultra in the xps2.