Get a firewire card.
Firewire support Isochronous traffic and USB does not.
Isochronous traffic means that devices can reserve bandwidth on the bus for periods of time and allows it to handle sound and video in realtime better. A basic form of QoS.
Also firewire typically use dedicated controllers whereas USB uses the CPU for handling the interface.
These are some of the reasons there are more feature rich Firewire audio interfaced then USB.
I would doubt that a Firewire to USB 2.0 converter (if it exists) would work well with audio interfaces - probably lots of lost frames (missing audio, etc).
Update: My bad USB does support an Isochronous mode.