The only requirement is that the card uses a OHCI compliant USB chipset since that is what Apple's drivers are designed to work with. Basically any USB card will work except for VIA, which is UHCI based (along with Intel, but they never made PCI USB controllers). The same restriction exists for USB 2.0 cards. While the 2.0 Hub is EHCI, the 1.1 part is UHCI on VIA cards.

http://en.wikipedia.org/wiki/UHCI#UHCI