If all you're looking to do is go spdif in, then just about any card with that feature should do the trick, since you're bypassing it's a/d converters anyway.
One thing you might want to think about, though, is whether or not it has word clock i/o. If whatever a/d converter you're using already has this feature, then it shouldn't matter if your sound card does. If it doesn't, then it might be a good idea to look in to a soundcard that does. Just makes everything a little more upgradable.