it depends on which card youve got by which manufacturer, the hercules card has it in the standard menu, for most of the others i believe that you would have to download it its almost pointless overclocking the clock speed, just pump up the memory and let it heat your feet
Alas, it sounds like you have upgraded to the Detonator drivers 6.48 and up. The overclocking tab has been removed from versions 6.48 (or is it 6.49?) and up. Even with the coolbits registry hack you can't get it. The only way to fix this is to replace these drivers: nvclp.dll and nvqtwk.dll then use the registry hack (coolbits).
From Detonator driver version 6.31 and under. Just get em from nvidia. I keep all my driver versions on my hard drive in folders- for cases like this. =)
You can use an external utility like Geforce Tweak. It is not dependent on which drivers you are using and is much more flexible. You can get it at http://www.guru3d.com/geforcetweakutility/