Ok, here's how I'd do it. If I was recording one to two tracks at a time, I'd use two unbalanced 1/4" cables patched from two mic channels insert points to two inputs on the sound card. Doing this is the shortest, cleanest path to tape, er, disc. Patching from the insert grabs signal directly after the mic preamp and before the EQ/fader section of the channel strip so you're not sending the signal through all the un-nessassary electronics of the mixer. You can then monotor in one of two ways. 1st is run the line outputs of the card to either two mono channels on the mixer or to one of the stereo input channels. This would be very flexable set up.
Now if wou want to record something like a live drummer with multiple mics, you're still restricted to two tracks at a time but you could patch from the main left/right outputs from the mixer, make a stereo mix of the drums from all the mics and send it as a stereo pair to the line input of the card. If you do this, you can't return the output of the card to the mixer as this would set up a feedback loop. You'd have to send the card output directly to the monitor amp/headphone amp/speaker set up.