A USB 2.0 PCI card is what you need. Best bet is any card with a NEC PD720100 or PD720101 chipset (xlr8yourmac results recommend the same). Read this for more feedback:
http://www.xlr8yourmac.com/firewire/fw_usb_card_deep_sleep.html
Stick with the NEC chip, and try NOT to use drivers, Tiger...