Seems like I have to accept the fact that the set up menue has to be operated through the S-video (480i) signal. As Capt says 'Bummer' as I need to switch inputs. However, I won't be watching the menue for hours at a time several days a week. Now for the XBox hook up details:

XBox to 990: Monster HD cable with RCA video signal (into COMP1) and optical fiber audio (into OPTICAL2).
990 to TV: HD-TV cable (audio and video [RCA]) from 990 HD out to HD-only component input (audio and video [RCA]).
XBox settings: widescreen (16:9), HD-signal [80p or 1080i])

Am I so mistaken to expect correct HD signal display on the TV? Let me know if you guys need more information. Thanks to everyone so far for their help!