Nick,
Your best bet to play import games is to buy a US Xbox. I know of a guy who gets them for UKP160 inc parcel farce delivery [the Sega/JSRF] box - I can PM you the details if you want.
Regarding Xbox PAL support of 480p.
If you plug a HD module into the xbox it will show up the option to set these modes, however, it was a bug. Its fixed in more recent versions of the Xbox hardware and if you install Xbox live you lose the options to set the modes.
Modding your PAL console is also an option. This will cost you +atleast+ 70 quid though.
However, MS are banning users who connect to Xbox live with modchips enabled permanantly, also several recent Xbox titles have had issues with mod chips, Steel Battalion being one of them. If you do decide to get your console chipped, ensure that they add an on off switch so you can switch the mod off to go online, but remember if you import any US online games you won't be able to play them online! [on of my modded xbox's is banned from xbox live so i know this to be a fact
]
THe best option really is to get a US xbox. [FWIW I have a US XBox, GCN, DC and PS-2, games are cheaper better quality and you get them sooner]. Metroid Prime on GCN is outstanding.