I used to be really big in the Halo 2 and 3 Montage scene.
The Hauppauge HD PVR, comes to mind, if your pc isn't that great.
With the Hauppauge and the right splitting, you can play on your HDTV and capture on your computer.
If your pc can handle it though... http://www.blackmagic-design.com/products/intensity/
You want to look into that. I havn't really recorded videos from my xbox in a year, but its a good start...