Well, there are PCI USB cards that say they work with Mac, but you could easily get a 'windows pc aimed' PCI USB card and get it working with the Mac if the chipset is Mac compatible with Mac, happened to me, but I had to return the PCI USB card since it was USB 1.1 and I was looking for a USB2 PCI card. So I found a PCI USB2 card that say it is Mac compatible and worked flawless, not that I use it that much, just to attach the iPod to the Mac.
People have had good luck with the PCI USB2 Belkin models, Allegro USB2 PCI card and with the The IOGEAR PCI to USB2.0 Card Model GIC251U.
The PCI USB2 card I got has six ports, four external plus two internal. The brand is 'Mentor', never heard about them, but the box says Mac OS X is supported. It comes with a cd-rom containing Mac OS X drivers, but I didn't need them. Just plug and play and it is working fine with Mac OS X 10.4.4. I tested it with a G4 MDD and a B&W G3, (both lacking USB2 ports from factory).