What is the BEST sound card to use with a MAC G5

Have a look at the Empirical Audio products, they sound way better than any soundcard I have ever come across - I am talking about the Off-Ramp Turbo. Since I know nothing about Macs I don't know if the wired off-ramp can work for you but the WiFi one will. Believe me, the Off-Ramp and a computer beats every CD transport I have tried. Running the off-ramp on battery power is recommended too.

There's a small problem with current generation soundcards. The best ones are designed for professional use and they are multi channel designs. They end up being more complex then any audiophile would require. That said you can get excellent sound quality from cards by Apogee, Lynx or RME.

To my knowledge there aren't any good sound cards for high end audio. Get a dac with either usb, firewire or ethernet. If you need spdif, something like the off ramp (usb=>spdif) would work, but direct usb is better.

What DAC are you currently using? I have done exactly what you are intending, ripped cds in apple lossless to a 750GB external HD and Mac iBook G4 which then goes via USB cable (Kimber) to a Paradisea + USB DAC then to preamp (Supratek). The sound is superb. You will encounter some here that say it isn't that easy, in my case yes it was just that easy.

Is there even such a thing as a sound card for Macs? It's all built in.