Someone help the man...
Seb, I'm not really geeked up but I've bought a HD LCD TV for between 500-600 quid and having seen xbox 360 running in high definition as opposed to standard definition I know that it will be s**t loads better. Much clearer picture.
With the Xbox 360 external HD DVD drive you can also benefit from super high quality HD DVD's.
The hard drive on the 360 stores your gamer profile, so all your results, your save points, your achivevements etc, as well as the live stuff, downloads, media you've copied over from your PC as well as loads of other stuff I'd imagine.