When Is A Soundcard Not A Soundcard?, interfaces Options

[Jaysee]

I'd almost given up hope of using my imac and ibook (both G3 and soundcar-less)and resigned myself to the expense of a new G4 or G5 for music.
After reading a 'head to head' comparison of audio interfaces in 'future music ' magazine, I'm starting to think that some USB audio interfaces are really an external soundcard in all but name. Have I got the wrong idea?
Would something like the tascam US122 or the USB MAudio audiophile enable me to get some use out of my present computers for sequencing virtual insrtruments? I'm not really interested in recording audio I'd just like to access the virtual instruments I read so much about.
I've downloaded a few 'demo's but most are mute on my current gear (though curiously I can hear a trial version of Absynth ok.....is this using the built mac 'soundmanager'?
Sorry if my question sounds a bit stupid but I've a long way to catch up in my understanding of the latest music tech acronym laden jargon.
BTW are G5's supplied with those special short soundcards built in or are they a third party product?