Most likely this is what you have, assuming nothing was changed after the original purchase.
http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&cc=us&docname=c00191338&product=93365&dlc=en&lang=enShould be a K6-2 500. A 'good' upgrade for that computer would be a K6-2+, a K6-III or a K6-III+, in that order. Even though it is a compaq, you should still be able to set a multiplier of 5.5X, giving you 550 MHz (max possible without overclocking FSB). To do that, there are probably jumpers or dipswitches on the motherboard for that. I've done the same thing to a similar Compaq once myself. Don't tell him he probably can't, because he probably can, even though it is Compaq.
The difference? The K6-2+ is a mobile version (lower voltage, cooler running) with a little bit of onboard cache (the K6-2 has none). The K6-III has more onboard cache, and is a desktop model. The K6-III+ is a mobile version with same cache as K6-III, but at a lower voltage. I have helped overclock one of these to beyon 600 MHz from it's original 400.
I'm running a rare special K6-2+ in my notebook computer (stock voltage is 1.65, normal is 2.2), it works great for word processing, basic internet browsing and runs BOINC when I'm not using it. Also I have the Garmin topo software installed, so I can connect my GPS to it on road trips and the such.