Good heatsink/fan is going to be essential if you want to overclock. Also make sure that you don't overclock the peripheral busses too much, like AGP and PCI. Going 100 -> 133 probably won't be too much of a problem, but don't be surprised if you run into stability issues.
If you're running PC133 @ 100 then you can probably bump the CL to 2 rather than 3. If you run into stability problems, though, push it back down to 3. I'm not sure if you'll notice much of a gain running with CL2 rather than CL3, but every little bit helps.
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