Hey Ho
Using your sound card as a starting point, as Dave says, it depends on your budget. If you can get one that allows for a flexible set up (multi DSP are best for this). A decent sound card will take a hell of a lot of load off your main processor, which can be very handy!
If you still want to create mixes using your external sources, or both external and software mixed get a soundcard that provides a break-out box for multi-channel IO (Creamware fits the bill, but there are others).
I agree, why mix to DAT? I route the output of my software mixer to the input of Wavelab, and use that for my mastering.Of course, you may still want to record at CD quality to get the most amount of tracks/effects/instruments, but that's another matter...
For a simple system, using no external sound sources at mixdown, mix and master internal to the PC.
Just to illustrate another approach, I use a pretty complex set up, with a high degree of flexibility. My current system uses a mix of live instruments, synths and outboard effects, plus computer generated instruments and effects.
Recording an instrument: I record through the mixer sub-mix bus when recording to PC (same for a vocal, any external instrument to be recorded), and into the stereo in of one of my Soundcards. If using a breakout box, I would use the individual channel outputs from the mixer to the BOB inputs. The recorded tracks are then mixed with any computer sounds in the PC mixer, and output through the same soundcard, back to the external mixer. The mixer then uses the main mix bus to blend the un-recorded external tracks (external synths etc) and output through the main desk output into the stereo in of my second soundcard (yet again this could be the same soundcard if you have enough IO ports). This signal is what is monitored. This all depends on a low latency system.
Mix down: External and PC generated sounds are mixed through the main mix bus on my desk, through a stereo input on my soundcard, straight to Wavelab.
Why such a complex system? It's very, very versatile. It make the most of the gear I already know and love, gives me the tactile experience of mixing using my desk. I also use an old VS880 as a midi controller for my PC mixer.
Sure it leaves the digital domain, but to be honest, for most people the loss in sound quality is not appreciable. I also know that if I want, I can easily set my system up for 96kHz, and mix everything in the PC. Being versatile is definately worth consideration.
Just some food for thought!
Cheers
John