Mac built-in sound (versus add-on cards), quality of analog-to-digital conversion? Options

[MikeR]

my question: What is the quality of "analog to digital" conversion by the built-in audio card on an old G4 Mac? Are there any tests that compare the conversion quality using the built-in sound and add-on sound cards? Are there tests that are technical (showing frequency response,...) or aesthetic (people with "good ears for music" listening and comparing in blind tests)?

why I'm wondering: I have a G4 (AGP, 400 MHz, bought May 2000) and I've been using it to archive old cassette tapes, converting them from analog to digital using the Mac's built-in 16-bit sound, using a Monster cable (RCA to minijack) to connect from the tape deck to computer.

my experience: Before I started archiving a previous set of tapes, someone teaching a "how to do it" class (who worked with PCs not Macs) said "you have to get an expensive sound card because the built-in will be very bad" and you'll get rumble, hiss, distortion,... But none of this happened with the Mac's built-in card, and there are no obvious artifacts. I'm wondering about the quality, though, at a higher level. I'm a semi-audiophile, not demanding "the best," but higher quality is definitely preferable. Would I get higher quality with an external sound card? If so, in what ways would it be better? And with which sound cards, in a reasonable price range?

MikeR