Eliminating jitter in a pc-based audio system

I've been looking at hard drive-based audio systems, and I saw that someone had picked up an apogee mini-DAC USB and Mackie 824 monitors, which seems like a great system to me.

However, I was wondering about the problem of jitter in the connection between the computer and DAC. Would it make a big difference to insert a jitter-reduction device in that chain? If so, are there any jitter-reduction devices that connect directly to USB?