Dual and single 1.6GHz 7447a G4 upgrades are your only new option right now. You can always comb through ebay for older faster upgrades like the 1.8GHz 7448 G4 CPU in my G4 tower.
Sadly NewerTech/OWC no longer makes the 7448 upgrades. The 7448 is easily the most powerful G4 chip ever made and was actually manufactured a few months after the last G5 was made.
Here is a link to OWC's CPU upgrade section with your 2 options if you want new with a warrantee:
http://eshop.macsales.com/search/MaxPower+G4