You probably have a socket 370 board, with that 466Mhz Celeron that came stock on that model.
HP says a PIII 800mhz is the preferred cpu for that socket type/FSB speed(100mhz).
Maybe you could upgrade to a cpu faster than 800mhz, but I'm not well informed on the limitations of your motherboard.
So someone will probably come along and help out a little more.