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.
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.