Aside from HDMI the following resolutions are possible with these connections (as long as your tv can handle said resolution on that input).
VGA:
HD DVD - 1080p resolution and all others
DVD - Upscaled as high as 1080p resolution and all others
Games - 720p games upscaled to 1080p, also supports native-1080p games in the future
Component:
HD DVD - 1080i resolution maximum, limited by AACS
DVD - Upscaled to 480p maximum, limited by CSS
Games - 720p games upscaled to 1080p, also supports native-1080p games in the future
Xbox 360 HD DVD playback: Maximum 1080i via component, 1080p VGA-only - Engadget HD