I will be nice and explain...
Your DAT player has a optical output on it, and that optical output jack is called a TOSlink connector. TOSlink connectors use a digital data transfer scheme called S/PDIF. This is a sort of "language" for transferring digital data between two devices.
To transfer digital data from your DAT player to your computer, you will need a soundcard that supports the S/PDIF standard. Not all soundcards do this. In fact, your average PC soundcard that usually comes with a computer probably cannot accept S/PDIF at all. So, you need a soundcard that supports it.
Soundblaster's Live has a S/PDIF in and out on it. This card can be got for around $60-$100, depending how much you shop around for it. That is probably about the lowest end soundcard you can buy that has S/PDIF in's and out's on it. From there, you only get into more expensive cards.
Anyway. Since the S/PDIF output on your DAT player is on a TOSlink connector, and most every soundcard that has S/PDIF uses a coax cable with a RCA connector for S/PDIF, you will need a conversion box that will take the TOSlink S/PDIF and make it into Coax S/PDIF. You COULD try to find a soundcard that has a TOSlink S/PDIF on it, but it would sure be a lot more expensive then one with a Coax S/PDIF on it, so the idea of buying a cheaper soundcard with Coax S/PDIF and purchasing a converter box to convert your TOSlink S/PDIF to Coax S/PDIF would get your digital audio into the computer for cheaper then buying a soundcard with TOSlink S/PDIF on it. Make sense? I am sure it does....
Let me tell ya a little secret though....
I would ditch the DAT player, sell it for as much as possible, and forget about a cheapy soundcard, and purchase a high quality soundcard and mix to the computer instead of DAT tape. I did this very same thing back in February and have not regretted it at all!
I purchased
a Lynx One soundcard (www.lynxstudio.com). This is a 24 bit soundcard and the fidelity of my mixes through this card are much much better then I was getting with the DAT player. Plus, the audio is on my hard drive, so I don't need to transfer anything to edit the songs, apply mastering eq and/or compression, etc....I can burn to my internal CDR burner with ease. It just makes sense.
Now, granted, there is a learning curve involved here. You will have to learn how to use some simple software to record the audio on the computer, and you will need to learn how to use some software to burn CDR's, but you will have to learn that stuff anyway because with the way you would be doing your transfers from DAT tape, you still have to have a recording software to do that.
An average DAT player is around $600 these days. They only provide 16 bit, up to 48KHz sampling rate audio (some won't record at 48KHz, and some won't record at 44.1!) But with the Lynx card, you get the benefit of being able to record 24 bit audio (waaaaaaaaaaaaaaay better fidelity then 16 bit) and the A/D converters (analog to digital converters) are of very very high quality on this card. I can assure you that no DAT player I have ever heard has as good as A/D converters as this soundcard does. NONE! So, you would be getting much better sounding audio using this card.
There are other soundcards that offer more then 2 inputs and outputs that the Lynx card does, and cost about the same. If you NEED more inputs and outputs then 2, then they may be the better way to go, but, you will probably be sacrificing the same high quality A/D converters with these cards that have more of them for the same price. Something has to give to make them that cheap eh?
The Lynx One card also has digital inputs that are AES/EBU or S/PDIF (coax) so you could still do digital transfers with it if you choose. In the case that you may have a lot of archieved music on DAT tape and you want it on the computer for safe keeping, you could rent a DAT player for a day that has AES/EBU digital outs and record all your music on the computer and make DATA CDR's of the .wav files for later use.
Anyway, these are just some things to consider. Good luck.
Ed