You can do one of two things.

1. Just run the "video out"
on the GameCube into the TV card.
Connect audio to audio in.
(generally need an adaptor for audio plugs)

2. Use a GameCube RF adaptor and connect
the the RF in on the tv card.
Tune in correct channel. (3 or 4)

3. Run the tv card's audio passthrough
into the "audio in" on your Pegasos
to hear sound if required.

You can use any regular video source as input.
I am using the RF adaptor for simplicity. :)