From the ProLiant RHEL support matrix
RHEL 6.0
RHEL 5.0
RHEL 4.4 (quad core processors) 1
RHEL 4.2 (dual core processors only) 1
RHEL 3.6 (Dual-Core Intel® Xeon® Processor 5100 Series)
RHEL 7 was never tested on this platform.
From the ProLiant RHEL support matrix
RHEL 6.0
RHEL 5.0
RHEL 4.4 (quad core processors) 1
RHEL 4.2 (dual core processors only) 1
RHEL 3.6 (Dual-Core Intel® Xeon® Processor 5100 Series)
RHEL 7 was never tested on this platform.