Well you can raise the FSB and / or change the multiplier. the 800 is probably locked, so you will need to close the L1 bridges on the cpu with a HD pencil. This will "unlock" the cpu and enable multiplier changes. Of course, a good cooler is an absolute must, and some case ventilation will help. Do so @ your own risk, Tbirds run hot and with a inferior cooler will have a short life span when overclocking.
Pentium 3 1000 @ 1100 ( windows 2000 server), Abit bx-133, 256 Mb Mushkin Rev.3 cas 2-2-2, ATI Radeon
AMD Thunderbird 1.4 @ 1.63 (24/7) IWILL KK266R , Corsairs pc 150 cl2, Leadtek GeForce 3