Older Xeons used to be to regular P4s what Pentium Pros were to Pentiums, very large on die cache, designed for MP support. The newest model looks an awful lot like a Pentium D to me.
There is probably better MP support with a Xeon. In the past to get more than dual procs you were required to have Xeon, that's probably still true.
As for 'true 64 bit', I think you're thinking of IA64 and Itaniums. The 64 bit support in new Xeons is EM64, Intel's implementation of AMD64.