computer isolation

Urban legend? I do not know if it will make a difference, but most all electronics are sensitive to vibrations as they have capacitors aka condensers- as in condenser microphones amongst other things.

Crossovers are micro phonic as well, just less perceptive as the signal is very small relative to the load. The Tannoy DMT series of studio monitor loudspeakers for instance. DMT was market speak abbreviation for Differential Material Technology. The capacitors were all wrapped in a butyl rubber type (mortite?) compound that had heat shrink tubing around it to help suppress the vibrations.

How much of this effects the output is up for debate. Acting as if it does not exist is a falsehood. Physics has not changed, only our applications of it. As for the OP's question, there are probably more important things to try to optimize in you computer audio rig.

Hotmail-

Is the g4 the aforementioned laptop? If so, try running on battery power

download "process wizard" or the equivalent and use it to renice (re prioritize) you processes.

Turn spotlight off

make sure that music files are not on the same drive as the OS

Turn off power saving and use a hot corner to turn screen off after you start tracks.

You may also try getting more RAM- the more the merrier for memory play. Be sure to use memory play and HOG mode along with "less is more". Playlists really help keep the tunes flowing with this.

I'd also try Ayrewave and Audirvana as players. I prefer Ayrewave at the moment, but it is a moving target.

If you haven't already, you might try taking a few choice cuts and make an AIFF and WAV copy to see if you can discern a diff.

It is chilly here in Michigan, I recommend a single malt scotch to make the left brain to right brain switch!

And by the by, I would not be surprised to find that you can hear a difference in isolation. The more involved I am with computer audio, the more I see similarities to analog. After all, digital waveforms are just analog square waves. If bits were just bits, then there would be no discernible difference between bit perfect player programs or USB cables. I am not saying you will hear a diff with isolation, but it would be foolish not to try if you think it may.

Now that I've thought on it, the players I mentioned require 10.6.