You can either scour the registry or use a tool like NVHardPage to add hidden options to the driver control panel (such as overclocking, AGP settings, etc.). After you've done so, head to the Display Properties > Settings tab > Advanced > GeForce4 Ti 4200 tab (assuming you're using the Detonator 4x.xx drivers), and you can enable support for overclocking via the Detonator drivers from there.
I also recommend using RivaTuner if you need to unlock some more "hidden" features.
The best thing to do is to increase the core/memory speeds in small increments each time, until artifacts show up or instability occurs. Keep in mind though, overclocking will decrease the life expectancy of the card.