16/44.1 is 1.4 mbps, 24/96 is 4.6 mbps. My recollection is that USB 1.1 is 10 mbps and USB 2.0 is about 40. So, I think USB 1.1 is probably OK for 16/44.1, but you might want USB 2.0 for upsampled tunes. I don't remember which interface the U24 runs at, but I have never had a USB data rate problem.
I think running things though USB hubs can also impact data rates negatively, as well as the number of other USB devices you are daisychaining. For example, if your are using a USB drive and a USB audio device through a hub, and pulling Apple Lossless files off the drive for playback, I think you will need about 1 mbps for the USB drive->PC and another 1.4 mbps for playback.
If you do go USB 2.0, make sure any hubs you have are USB 2.0 compliant; its downwards compatible, but I think that means you end up with the lowest data rate in the chain.