The FW410 has facilities that make recording convenient. No Mac comes with these features built-in. The FW410 will sound better than the analog circuitry of any Mac.
G5s natively handle 48kHz/24bit audio - whereas earlier macs topped out at 44.1kHz/16bit. Can you hear the difference through the analog output? I don't think so.
Any analog audio circuitry that exists inside a computer (built-in or PCI card interface) is highly susceptible to electromagnetic radiation from other computer components. This results in a variety of artifacts (clicks, thumps, hum, broadband noise) added to the analog audio signal. This is perhaps the "crap" that your salesguy was referring to. But this would also apply for the analog i/o of the G5.
However, high quality playback from a G5 is pretty easy to achieve if you have a digital receiver with an optical input. Just make the optical connection.
To achieve a similar quality of output from any other Mac, you really need something that bypasses the built-in analog circuitry.
You still need an A/D device to take advantage of a G5s digital input - and these tend to be in the same price range as the FW410 anyway.
That being said, if recording or listening is taking place in the same room that the computer and/or fishtank lives in - then the choice of "built-in" audio versus Firewire interface may not be the most important factor determining the "quality" of recording or playback.
Furthermore... "quality" is subjective - and if listening to a CD from the analog output of a Mac sounds great to someone - then it does indeed sound great.