datfly -- any reason that the interface needs to be external? While a computer's a bad place to perform analog>digital conversions, it's not really a bad place for digital data to be received.
So, unless all of your computer's internal slots are already in use, going internal could be a more versitle option.
Regardless, there are good choices for both.
Internal, lots of folks have had very good luck with the M-audio audiophile 2496, it's been proven bit-perfect (i.e. it will accurately record the digital data passed from your dats via its coax input -- note, no optical in on this card) and is relatively cheap, you should have no problem finding one for under $100. There are other cards that will do the same thing at similar, or higher, prices, but given its userbase and proven track record, the audiophile 2496 is probably the safest choice as an internal card.
External -- much more limited options. First off, you'll need to have a usb2 (and not older usb 1.1) interface on your pc. My understanding is that the 2 cards to look at here are the m-audio audiophile usb ($1xx), which will allow for coax in, and the m-audio transit which only provides optical i/o, but runs under $100.