I have the Waveterminal U24 and am very happy with it. It is a USB 2.0 device but is backwards compatible with USB 1.0. I'm pretty sure most USB2.0 devices are backwards compatible. My Macs are all USB 1 and I've never had any problems streaming audio with them. USB 2.0 has a significantly better data-transfer rate, but in my experience with Mac/Waveterminal/iTunes it makes no difference. If it is not streaming music fast enough you would get pauses in the music.
Marco