It depends on the USB specifications of the card. Newer PCI cards support a newer PCI standard (sorry, I don't know the version numbers) that enable them to work fine in PCI-X slots.
With this sort of thing I always check
http://www.xlr8yourmac.com There's always a wealth of information on there with all the gory details.
Worth noting that I bought a Belkin 4 port USB 2 card and that installed fine. Some problems with it though - Any devices plugged into it on startup have to be unplugged and plugged back in before they work. With some devices connected when you put the computer to sleep it wont wake up again, or after several minutes the mac goes into 'Take off' mode where fans blow at full pelt, requires a hard reset.