If you don't want to spend too much on the HD PVR, although i will admit that it records beautifully, you should check out the Roxio Game Capture Card.
*This is not as good as the PVR, just a cheaper suggestion*
It records at 480p, but if you have skills with video editing this card gives you enough to work with to make the video seem like 720p.
http://www.roxio.com/enu/promotions/landing/machinima/game-capture/console/overview.html
That link is 70 dollars instead of 100.