please don't take this question the wrong way, but may i ask why you're looking to get an external soundcard?

the reason i ask is this: the specs you're looking for (digital in/out) + your quoted budget (around $150) might make it tough to find a decent external soundcard. sure, there are cheaper external soundcards out there (e.g. Edirol UA-1D) that have digi-ins/outs, but the cheaper ones are not bit accurate... which means the incoming audio stream is resampled. (a big NO-NO.)

i've read that some pricier external soundcards are bit-accurate, but they are way out of the price range you're looking at, e.g. $300+.

are you wanting a soundcard to transfer DAT>digital cable>hard drive? or something else? if this is the case, you could probably get away with buying a used Nomad Jukebox 3 off ebay within your budget, and it has the connectors you would need (optical input, and USB/Firewire output to a computer). although, i'm not certain if it performs bit-accurate transfers...

on the other hand, you can get a nice *internal* soundcard with analog and digital-ins/outs that is bit-accurate for much, much less... well below $100 (brand new). and on this side of the spectrum, please stay away from Creative Labs SoundBlaster soundcards, as most of them resample digital signals.