Peter_s is correct - USB 1.0 is more than adequate to handle 16 bit 44.1 KHz data streams. USB 1.1 handles 12 MBits/sec and cd-quality audio requires 705,600 bits per second (16 * 44,100). 24 bit 96 KHz requires 2,304,000 bits per second so it, too can easily be handled by a (dedicated) USB 1.1 interface.
Where things get complex is when you're sharing devices on a single USB bus, such as an external hard drive (note that most computers have a single USB bus with multiple ports - i.e. two USB ports does not mean you have two USB buses). USB 2.0's additional bandwidth will help here and prevent data choking/collision.
The other area that a lot of these external audio interfaces is used in is for home studio recording, where you (may) need multiple channels of audio running simultaneously. USB 1.1's limited bandwidth chokes on 4 simultaneous channels, which is unacceptable in many situations.
With all that said, why even bother paying more now for an interface that can support USB 2.0 or 24/96? When the 24/96 formats come out for software they will surely have some form of hardware DRM (digital rights management) that the current generation of audio interfaces don't have.
