The Harpertown x5460 was the fastest tested processor for this server; Harpertown Processor  Part # N856G SVC,PRC,X5460,1333,XDH,E-STNP.  Anything beyond that was untested by Dell.  As theflash1932 explained, you will need to have matching processors and the system will need to be completely updated. Regards,
What is the list of CPU's that a Poweredge 2900 can be upgraded with? How many?
Poweredge 2900 Series III CPU Upgrade