  This paper describes updated predictions, as a function of the underlying
cosmological model, for a serendipitous galaxy cluster survey that we plan to
conduct with the {\em XMM-Newton} X-ray Satellite. We have included the effects
of the higher than anticipated internal background count rates and have
expanded our predictions to include clusters detected at 3 sigma. Even with the
enhanced background levels, we expect the XCS to detect sufficient clusters at
z>1 to differentiate between open and flat cosmological models. We have
compared the XCS cluster redshift distribution to those expected from the XMM
Slew Survey and the ROSAT Massive Cluster Survey (MACS) and find them to be
complementary. We conclude that the future existence of the XCS should not
deter the launch of a dedicated X-ray survey satellite.
