I found this: http://www.xboxcorp.com/faq.shtml:
    Does the Xbox support HDTV?

    Xbox was designed with high-definition TV support specifically in mind. Xbox supports HDTV and TV resolutions up to 1920x1080 with the purchase of the High Definition AV Pack. The High Definition AV Pack supports HDTV RGB video and your choice of RCA Audio or Digital Audio over optical cable.
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Bitt Faulk