RHEL5.x virtualization relies on etherboot for remote booting. Etherboot is an implementation of the pxe standard, but lacks some features that are present in the new gpxe boot technology which is not shipped with RHEL.
It is possible to use the gpxe roms with RHEL 5.4. As an example, gpxe roms can be used to interpret requests generated by Microsoft RIS or WDS.
All components present in RHEL5.4 are capable of booting gpxe roms. The roms
can be obtained directly from http://rom-o-matic.net/, or other sources like
the Fedora Project.