If your motherboard supports a 100FSB, you might try looking at the lower to mid-end P3 based Celerons, but the real question is this: Does your motherboard support the Coppermine core? If so, you can go with a Celeron from 600-1.1 Ghz (or P3 from 500-1.1Ghz).
To that effect, I've found some various websites that say the FSB can actually go to 133MHz (not supported specifically by the manufacturer, but very possible). I also found a converter that will allow you to plug a socket 370 P3 into the PPGA socket, since it can't directly be done (both P3 and Celeron are socket 370, but apparently the 370 pins vary in function between them). With one of these, speeds of 800Mhz P3 have been done safely.
Okay, so you do have some room to move up, but I'll stop my research at the first two google hits, you can do the rest...
All these CPUs are cheap if you can find them (~$50 retail, less on eBay), so you might as well go for whatever the highest turns out to be, you just may have to search for what it is.
Me machine in me profile.