As far as processors that Apple used in the G4, your 500 MHz is the limit for the AGP Graphics/Sawtooth model. Sonnet has some G4 upgrades though that you may be interested in. Their webpage is here: http://www.sonnettech.com/product/encore_stg4.html
Shop for your Apple, Mac, iPhone and other computer products on Amazon.
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.