Pdreher:
There are several common methods of transmitting the signal to the DAC. A good asynchronous connections allows the DAC to control the signal flow, not just "catch" what is sent by the transport mechanism. It will greatly reduce the net jitter. You might be amazed at the palpable difference it makes. Even legacy DACs are often invigorated with a device as simple as the hiFace. There is one listed here at the moment. Otherwise it is at Tweak Geek. About $150-$180.
Some other examples:
Firewire DACs: Metric Halo, Weiss DAC2 & DAC202- All three are excellent.
USB DAC: Wavelength products, Ayre, DCS- possibly HRT music streamers
USB>Spdif: M2tech hiFace (they have some new items coming out that should be hot too), A.R.T. Legato, Halide Design Bridge
If you still have a DAC around and a Mac computer I highly recommend trying the hiFace (works with windows too). For the money I think you might really be surprised. It helps even something like a Benchmark DAC1. Use a nice digital cable if you have one. Otherwise connect it right to the DAC and use a USB cable to it. There is a lot of info on the Computer Audio Forums.