AFAIK Pinnacle Studio MovieBox HD can only record SD videos from xbox360 (let me know if you manage to record HD videos from xbox over firewire). For the best HD video capture see the following thread:
How to capture/record video from HDMI (record game play from XBox 360, PS3 to PC)

See also
Blackmagic Intensity Pro issues when recording from xbox 360: Audio and video problem
for more tips on capturing from xbox 360: