I use a Dazzle Hollywood DV Bridge (firewire) to capture my Xbox 360 gameplay using Kdenlive. I record commentaries using Audacity and lay the audio over the video. Here's 1 I did recently where I stretched the 720x480p source to fill 1280x720, it turned out ok but I really want to purchase either a Black Magic Intensity Pro or an HD PVR (both work in linux I've read).
http://www.youtube.com/watch?v=g8-QXpVowXk