Good afternoon, I've run my music out of mac's only. In order of progression, I've used a B&W G3, a D/A G4, a quadnostril G-4, a Mac Cube, a G4 Mac Mini, a dual 2.0 G-5, a 2.66 core duo imac, and presently run it out of a 3.06 iMac core duo. Other than noise from the computer itself, e.g. the quadnostril, I haven't noticed a significant audio quality difference among these. As the processor speed and amount of RAM have increased, I noticed a more fluid networking experience, but not a noticeable difference in sound quality. However, I also have 56 year-old non-golden ears, so YMMV. Good luck. Cordially,
BTW, the iMac generation that was just superseded this week by the new models was whisper quiet, reasonably powerful and the Apple Store's $999 price for a 20" 2.66 iMac would be awfully tempting. i have no affiliation with Apple Computer other than as a customer.