USB 2.0 shouldn't make any difference, as 56,000 bits/sec is much slower then the 4,000,000 bits/sec that USB 1.x provides.
What about an external serial modem? They're probably cheaper. I just bought a US Robotics 56k v.92 external that I use on both my desktop and laptop. Not the most portable device in the world, but it's a great modem.