Certified servers/workstations from Dell that are equipped with 6-core AMD Opteron processors from the 2400 and 8400 series (codenamed "Istanbul") require, at minimum:
Red Hat Enterprise Linux 4.8 (for those certified to run Red Hat Enterprise Linux 4)
Red Hat Enterprise Linux 5.3 (for those certified to run Red Hat Enterprise Linux 5)
Please see the Red Hat Hardware Catalog at http://hardware.redhat.com for a complete list of certified systems. Note that the requirements for Istanbul may necessitate the use of a newer update release of the OS than the one used during the original system certification.