I've had a lot of trouble with P3s and slockets. I had an Abit BX rev 2, which is supposed to support a P3 with a slocket. I tried a celeron 533 FC-PGA, and it was very unstable - I ended up having to buy a new motherboard. I am running the Celeron 533 at 800 Mhz, so it ended up being a good deal anyways (especially 8 monthes ago).
If you only have PC100 memory, I would go for a Duron or Athlon.
At my favorite store(
www.mwave.com) a motherboard bundle is:
EPOX EP-8KTA+ Motherboard BUNDLE W/ COOLING FAN, 800 MHz Duron: $ 191
This MB has all the overclocking features (cpu multiplier, voltage, etc...)
If you have PC133 memory, then you could get a Pentium3 800EB Mhz, which is ~$200, plus the price of the slocket.
A P3 will be a little faster in games than a Duron, but the Duron won't disappoint. You'll be able to upgrade the Duron to an Athlon in the future for low cost.