333mhz?
My old shitty PC ran a 670mhz processor! What chip you running?
I haven't actually ever overclocked a CPU, but to be honest, the advancements you'd get doing on a 333mhz is gonna be minimal at best!
If you can find out what socket your motherboard has, you might be able to source a P3 chip for it? I have a P3 (650mhz) mobo here and I was gona build it up and over clock it to 1000mhz, but got pissed off buying shonky used PC bits and bought a non-shonky used PC instead

Still for £200 I got a nice Dell with a 2800mhz Pentium 4 CPU, 1.5gb ram and a biggish HDD