It may be possible to get an Intel logic board in there...but from my experience, an Intel mini logic board costs almost as much as a brand new Intel mini itself, so it's not really worth it.
But, even though I have a 1.83 Ghz Intel mini, my daily computing is done on my iBook G4 1.33 GHz, and it's definitely sufficient for basic needs.
Max out the RAM, get a faster hard drive, and your mini should be pretty speedy. 🙂