Hi there!
Just a few words to those wondering if the 360 is worth buying if you don't have a high defenition telly.
The 360 works fab on any standard TV made during the last seven years or so. As long as your telly accepts a 60hz signal, you're good to go. This is pretty important as some games will not be playable in 50hz (techy and boring, but it's all to do with screen refresh rates).
If you're planning to use the 360 on a standard telly, the AV cables you get with either the core, or premium system, won't do it justice.
For best performance, you should get yourself the official microsoft scart cable, which carries and RGB (red, green, blue) signal. This will give you a gorgeous picture - the best you can get on a standard TV. Trust me, the picture still looks amazing and, although Hi-Def is better, you'll still be beaming.
If you still crave Hi-Def, you can always hook your 360 up to a computer monitor with the official VGA cable. However, I really wouldn't be too fussed if your TV is 28' inches or lower.
Thanks for the memory?
Well, if you plan to connect up to xbox live, then a hard drive is the best option, as you'll be able to upload your gamer profile and download updates for your console (they do this a lot; very handy, although game-specific updates, apart from those designed to fix bugs, usually cost "microsoft points", which you can pick up at Virgin or HMV). Xbox live is top banana - especially if you play co-op with friends!
If you're not venturing online, then a memory card will work just dandy and will save a lot of game data (achievements, settings, etc.).
Hope the above helps! :-)