You don't need a converter for VIDEO if you use the composite cords. You aren't going to get the better resolution on a 2005fpw.
And yes, you can buy adaptors for the red/white audio cords to turn them in the typical "line in" cord for a sound card. One came with the 360 VGA cords and I bought one at Radio Shack this summer.
Edit: If you're going to a receiver, then you shouldn't need any kind of adaptor on them...