Yes sorry missed that it was late. Even the G4's like the quicksilver had to have the sonnet firmware to use accelerators. In fact I had just done that last year with a 1200 MHz sonnet upgrade in my quicksilver.

Just FYI search function here is your friend. Lots of links and all these common issues have been hashed out.

Yes your old CPU will work with the sonnet firmware, in fact the sonnet upgrades say keep your old CPU for possible sonnet CPU failure. (Troubleshooting and such)