Importance of source quality with asynch USB DAC?

I have never tried computer audio and I'm considering either a great DAC with mediocre synchr. USB in, such as Bel Canto DAC3, or an asynchr. USB DAC such as Ayre or Wavelength. The advantage of the former being I can use my CD player as source until I can buy a new computer, and the advantage of the latter being I can simplify the system.

So my question is how important is the quality of the source computer with an Ayre QB-9 or Wavelength. My current laptop is a Dell Latitude 420 (I believe), which is 4 years old. Alternatively I could use an even older Sony Vaio PC, but that's old. I know people rave about Mac Mini + an asynch USB DAC...that would be my goal down the road, but how would it sound in the meantime, compared to a Bel Canto DAC3 fed thru S/PDIF from a Rotel CD player as transport?

Thank you!