I tried both Google Desktop and Coppernic before. But I prefer Coppernic because it is not browser based. In terms of resources, I didn't notice any performance issue while using this two desktop search. IIRC, the Google Desktop doesn't update the index as often as Coppernice. ie: If I moved my file, GDesktop will still display the old location while Coppernic will usually display the latest location.

Since I'm switched to Vista, I'm currently using the built-in desktop search function which perform pretty good.