Try downloading Belarc Advisor. It should give you the chipset information necessary in order for us to provide you with an accurate answer. It can be downloaded here:
http://www.belarc.com/free_download.html
It will also come up with oodles of other information, but the Chipset info is most important. Browsing through the docs on HP's website, I was unable to figure this out myself, or I'd probably already have an answer.
Chances are, though, that even the fastest chip this motherboard will be able to handle won't give you what you want from this system. Don't expect an old Socket-7 system to be able to do much, and the money spent on the new chip (whatever chip that may be) may do better to be saved for a new system entirely.