> There's no such thing as a G4 PowerPC CPU that goes past 1.5GHz.
> So that idea is out of the door already.

Even if that was true (which it is not), it would only answer the "2GHz" part of MorphDelf's question for "1.33GHz, 1.5GHz or 2GHz CPU cards".

> No, "Mac overclocker cards" are not a decent reference for this

And what about Apple PowerBooks G4 with official 1.67 GHz? Or simpler: What about Freescale's listing of "G4"s above 1.5 GHz? All made up nonsense?

> they're exactly that. Ridiculously overclocked.

You're right for the 1.8 GHz (up to 2.0 GHz, I believe) G4 upgrades.

Edit:
And interesting regarding Genesi's own opinion about the existence of G4s above 1.5 GHz (from the Wayback Machine link above):

"Freescale 1.7GHz upgrade program will be available later for an additional $49.95."

[ Edited by Andreas_Wolf on 2009/2/21 7:22 ]