you need some more details. His computer could either be a socket 478 or 423. If its socket 423 you can cut out most upgrades but if its socket 478 it could support up to 3.2ghz and higher. However it is doubtful that the motherboard will support the 800mhz fsb so im thinking mabye 3.06ghz 533 or 400fsb cpu will be the best one it could support. This is just an educated guess so post more details and ill be able to give you some better answers.
to my knowledge, hp doesnt lock the cpu upgradeability of their machines.
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.