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Potentially. The last post in this thread has a statement about a user using a Dynex card.

To be more specific, you need an Open Firmware card. Many windows machines use BIOS for their communication; Apple now uses EFI, but for Motorola/IBM architechtures, Apple (and others) used Open Firmware.

Personally, I would head for the Sonnet, but that's just me.
 
ANY USB card with a NEC chipset works in the PowerMac G4s and G5s (had one in both myself). This has nothing to do with Open Firmware...

This holds true even today: Bought a cheap PCIe USB card with a NEC chipset for my Mac Pro – worked out of the box under Leopard as well as Snow Leopard.
 
ugh

That card just from the best buy pic has the VIA chipset, budget level USB controller

Mac driver for VIA available here:

http://www.viaarena.com/Driver/VIAMACUSB_160_G4_p1.pkg.zip

Only has support for up to Tiger, only supports PPC G4, Leopard and higher support unknown

Here is a cheap USB card for $10 that has a NEC chipset and will work with Mac:

http://www.newegg.com/Product/Product.aspx?Item=N82E16815124008

Look in the customer reviews, was installed on a PowerMac G5 with Leopard and it worked
 
USB Cards

I have a PowerMac G4 Quicksilver w/a Sonnet 1.8GHz CPU, and use Belkin 2.0 Five-port PCI USB cards that I've purchased thru Staples. I have had no problems at all, use my HP printer, Dymo LabelWriter, mouse, keyboard, Apple 23 HD Monitor, LaCie Mobile Drive, and Palm T/X hooked up to these cards. I've used Belkin products over the years and recommend them.
 
Pci Usb

I picked up an ATEN 2-Port USB 2 card for about $10 used on eBay. It is working like a champ in my Gigabit G4. NEC-chipset based, no drivers needed. I tried a MacSense card 5-port card prior, and it did not work.
 
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