Lane,
You're going to have the Fostex connected to speakers right? And those speakers would probably sound best if they were connected to a stereo system of some sort which means a receiver is involved. Most receivers allow you to switch the output between phono-tape-CD-Aux (etc.) so your sound card (on the back of the computer) should have the rear stereo out puts go into say CD on the receiver. And your Fostex rca stereo outs could go into the AUX input on the receiver. From there, it's just a matter of running the y-jack from the headphone out on the receiver to the Mic/Line 2 input on the Live Drive.

Just make sure when you're listening to anything while the Y-jack is still connected to Mic/Line 2 that you don't select CD output on the receiver or your going to get some real howling feedback. The Mic/Line 2 input on the Live Drive should just be reserved for the Fostex and the AUX out on the receiver, and you should smooth sailing. The recordings I sent you were all captured this way to the computer. Pretty hassle free set up and decent enough sound.