I finally found a thread cleared this up for me. Stephen Neal on the AV Forums wrote this about the RGB Full setting on the PS#:
I think people are confused as to what FULL and LIMITED RGB are for - they are NOT to do with Deep Colour and xycc colour.
They are to do with support for legacy RGB DVI displays which support only 0-255 RGB levels and not the broadcast/studio standard of 16-235 RGB levels which almost all HDMI displays should support, and is the levels standard used for DVDs and BluRays.
Many displays will cope with either setting and deliver near identical results - though will need re-calibration depending on which one you chose. If you do not recalibrate, then FULL will appear to have deeper, crushed blacks and thus more saturated colours, and brighter whites. However with correct calibration on a studio level capable display there should be no difference. Additionally Limited levels will allow blacker-than-black and whiter-than-white to be passed through, whereas Full levels can't allow this as anything below 16 or above 235 in studio levels terms is mapped to 0 or 255 in Full (aka PC level terms)
AIUI Super White is the option to allow Whiter than White to be passed rather than clipped - and as such is only available in YCrCb (and possibly Limited RGB options)
From this I can assume that my year-old HDTV does not need the PS3's Full RGB setting and I should use Limited. I calibrated my TV with this setting set to Full using DVE on Blu-Ray, so I can also assume I need to recalibrate with the Limited setting.
That should make a difference with the Xbox 360's picture. I think I'll be trying Auto or Source combined with Standard assuming the Xbox outputs video in the same manner as the PS3.