"Creative Labs" has always built a decent sound card, as does
"M-Audio" and "Yamaha". Some Captivate users have reported
difficulties with USB audio cards, so if that's your pleasure,
trial and error might be the order of the day.
If you are using a PCI slot card for an internal
installation, note that caution should be taken to avoid conflicts
with audio built into the motherboard - though this may not be
necessary (or applicable) since your interest seems to be with
external, not internal cards...