JediTray said:

Looks like most of the external USB sound cards only have a 3/5mm input, and are 16 bit.

6.3mm jacks are basically the music industry standard, so many of the more expensive soundcards will favor these connectors, cause they're mostly used by musicians...
But you can indeed get soundcards (USB also, or firewire) with any resolution, it's just a matter of money..

So I'd just be spinning my wheels that way too. Instead of turntable-mixer-internal sound card, it would be turntable-mixer-external sound card-computer.

The sound is digitized in the soundcard, so any external soundcard->computer connection is purely digital. The idea with an external soundcard (or one with a breakout box) is simply to eliminate any noise/interference that might come from the computer itself. But to be honest I don't know if this is really a problem or is more academic? Has anyone measured this supposed interference on their soundcard? In any case most semi-pro soundcards are internal with external breakout box.

I don't know your budget, but here is one USB and one PCI soundcard that are (relatively) cheap and should be able to do what you want: M-Audio Delta Audiophile 2496 & Roland UA-1G.