I would really recommend getting an HD capture card. Non HD gameplay is PRETTY lackluster to watch, no offense. I will not and cannot watch non HD Let's Plays. If you're going to do it, I recommend doing it right. Go HD. Even 720 is good, just don't get anything lower than that.
This is a nice one.
http://www.rakuten.com/prod/elgato-game-capture-hd-playstation-3-xbox-360-recorder-for-mac-pc/231935533.html?listingId=-1
Supports 1080i and 720p res.