Do I need to buy the MS Advanced HD Pack in order for the Xbox to output HD? I have a modded UK box and have switched my box to NTSC and enabled the HD resolutions using Enigmah, but only have a cheap compatible component cable (rather than an original MS one).
All the games I've played so far still look the same of mixture of jagged lines and poor resolution on my 32" Samsung LCD TV

Maybe I've just not tried one that supports a HD resolution? But knowing if I have the right cable or not will put my mind (and my wallet) at rest

First be sure that your game really supports 480p and make sure that it is enabled in MS Dashboard (the options will only appear when your HD-pack is connected and when your Xbox is using NTSC). Load the game and compare it to S-Video. It should look better...