Right. This is how I see it.
I'm going to assume that your Xbox is in NTSC mode, as HDTV is disabled when set to PAL. Sadly I can only say I've tried the opposite of your scenario; NTSC games seem to default to PAL 50. Now most games should run perfectly when forced to a different Hz (and I'm talking about NTSC games only for 60Hz being run in 50Hz, your PAL games will be partially 60Hz compatible). However SOME games seem to have problems. NTSC Megaman Aniversary collection runs slower in regular PAL, and I assume if the PAL version isn't 60Hz compatible (if there WAS a PAL version), it'll run faster in NTSC mode.
NTSC games will work 100% of the time, but most PAL games won't have a problem. Don't hold me to that though =P