I currently have my working computer overclocked to 2.8 from 2.2 ghz. The Radeon 9800 that I have in there is also core overclocked up about 150 Mhz.
My real OCing is done on my "clunkers"- a collection of pentium 3's and twos that I have sitting around. I have one of the celerons overclocked from 500 Mhz to 933mhz, which is a bit of a jump for that old of a processor. The rest of the clock jumps are pretty low... in the 20~100Mhz range, but I can still say that they are overclocked
Right now I'm kinda getting into overclocking the older ones by jumping pins on the processor, but I haven't had any success stories yet...
We'll see.