I would personally overclock a 1.8 simply because the system will be worth not a great deal....
If its a 1.8 Intel run a big fan and jack the FSB as high as you can.
if its a AMD unlock the processor, drop the mulitplier a little and jack up the FSB to increase bandwidth across the board.. I would also personally run a large heatsink on this and overvolt it a little.

The stability of these systems overclocked would by large depend upon the quality of the components, specially the memory and PSU

The big question to ask in reality is can you afford to kill the system? - over clocking does stress components and if you have cheap shit it will fail when pushed

1.8ghz systems clocked on air generally make around 2.5ghz in my experience with the right bits