The fastest PGA478 CPU was the 3.4 GHz Prescott (if my memory serves me):http://www.xbitlabs.com/news/cpu/display/20040123103754.html
We have a 3.2 GHz Prescott running fine in that same motherboard,
and we dedicated that system to a backup storage server,
which we turn ON only long enough to do backups:
this results in prolonging its useful life even further.