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Theoretically, you can overclock any processor.
Some will go much higher (in percentage) than others, with the "norm" (if there is such a thing) for AMD's about 25%, Intel about 40%, Cyrix about 10%.
Some AMD's (Duron) will go much higher, but heat is a huge factor.
With peltier coolers, liquid immersion cooling and such, you could get a possible 100% increase.
For example, a Celeron 500 with a 66mhz bus, may actually run, with enough cooling at 133mhz bus. The most I've ever got was a 120mhz bus on a 600 fcpga.
On the PIII, (100mhz bus) it's common to run them at 133mhz bus and I have a 600 running at 150mhz bus.
Stability of motherboard and RAM are huge considerations. The Intel BX chipset probably still rules as the most overclockable chipset (Asus P3B-F for example).
Don't try this on your Compaq, Gateway, or SIS chipset board.