The p3 may outperform the celeron, but you won't see any difference with such a minor difference in clock speed.
I have an HP 7840 with a trigem cognac motherboard and an i810 chipset. I'm running 256mb pc133 memory. My OS is ME. I currently have a 766mhz celeron processor and would like to swap it out for a 800mhz P3. The mobo has an 800mhz max and 512mb max for memory. I'm trying to get the most out of an outdated system. Whats the best I can do without building an updated computer?