Unfortunately being anywhere other than in the Americas or Japan (NTSC countries) sucks as far as HD xbox gaming goes.
As far as I've seen when looking at game covers there is no 480p, 720p or 1080i features in any PAL games, only on NTSC ones.
You can do as stated above and switch your xbox to NTSC and enable these modes in the dashboard once you have a HD cable plugged in, but as far as playing games you've backed-up from PAL discs in Hi-Def, I'm not sure.
If this is the case you may may find that the games only run in 480i.
To get them in Hi-def you will need to download yourself an NTSC backup of your games.
I don't have a HD ready screen yet other than a 21" monitor, but I need to get my VGA mod up and running or make the Component to VGA transcoder I've been designing before I can use it.
I hope I'm wrong about the HD limitation with PAL games as I will be facing the same problems as you as soon as I do get onto HD.