[lordsmurf]

You really don't want to change PAL to NTSC, or NTSC to PAL, if quality is important.

You can mix resolutions by putting each in a separate VTS (video title set, sometimes called "track"). Crappy low-end software often won't let you do it, but better software will.

PAL and NTSC cannot be on the same disc, it's not DVD-Video compliant. You have to ignore compliancy restrictions (if the software allows it -- most does not, and for good reason). The disc won't play on most DVD players.