There isn't a huge amount of difference between the 3 packs available at the moment.
The Arcade is the cheapest pack - but it doesn't come with a hard drive or wireless controllers.
The Premium pack has a wireless controller and a 20GB hard drive.
The Elite 360 has a 120GB hard drive and an HDMI port. This is only useful if you have an HD compatible TV with an HDMI slot or are planning to get one.
Xbox Live Silver comes with the console anyway, this allows you to connect to Xbox Live and download demos, arcade games, movies and music videos. Xbox Live Gold costs �40 a year and will allow you to play multiplayer games with other people over the internet including footie :)
If you want to download anything, the Hard Drive is essential. it works out cheaper to buy the Premium or Elite pack rather than the Arcade and purchasing a separate hard drive.
My recommendation would be the Elite pack, it gives you the option to download games should you so want to. I think the Elite is overpriced. for any more information, shell out �6 on the Official Xbox 360 Magazine as each issue has a bit about the differences between each pack and a list of features and how to use them.
Hope this helps