Yeah, you got it right.
USB Soundcards can be just as functional as PCI cards without the need to take your case apart for installation. Because most motherboards come with good onboard audio as a standard, no one really uses PCI soundcards except musicians (and maybe you and me).
I haven't really used a USB soundcard but I know of IceMat's range and also a number of USB headsets that integrate a DSP sound chip and a headset so you can plug it in and listen without a separate soundcard. Plantronics has a couple of nice ones (DSP-500, DSP-400?)
If you're into music I would recommend National Instruments' Guitar Rig v2 which comes with a controller device that doubles up as an excellent - and I mean excellent - USB Soundcard. Device ~ US$250, Guitar Rig Software ~ US$500.