"Since Mac OS X 10.3 was released on Friday, October 24, 2003, there have been many reports of the NVIDIA GeForce4MX cards (and other cards from different manufacturers) not working under this new operating system. It seems that Apple pruned out any extraneous code it could from different .kext files (drivers). What is strange is that only customers with Power Macintosh G4 "Sawtooth" logic boards seem to be affected by this loss of video - Gigabit Ethernet, Digital Audio, Quicksilver machines are fine. It seems that the lack of the 28v ADC power connector on the logic board causes the card to not be recognized. After all, Apple did not ever ship these cards as a part for older systems, and NVIDIA has never retailed cards for the Mac platform for end users to buy. The only way to get the card was to receive the card bundled with your system or to purchase one from a 3rd party company such as OWC, and it's up to Apple to write the drivers for the card as the OEM vendor of the product.
As we have been selling these surplus cards for many months with no problems until now, we have been actively pursuing any leads on restoring functionality to these cards. After all, they are still working in most systems including ones they originally shipped from the factory installed in.
There are a number of existing forum threads on Apple's own discussion boards about this, as well as reader reports at XLR8YourMac, MacFixit and MacInTouch. You may find them interesting - I know we have been reading them actively.
This fix does have one limitation: Quartz Extreme is NOT activated but the card is at least usable while we continue our efforts to restore full functionality. We want the cards to work 100% as much as everyone else does :>) With any luck there will be a follow up article to this one in the near future with Quartz Extreme support, or Apple will fix what has been broken with a 10.3.x update."
more on the site
These cards are pretty popular upgrades - luckily it's just the Sawtooth - Cube and Gigabit and QS owners are okay.