If your looking for a basic xbox 360 console that you can upgrade to as you wish then the xbox 360 arcade version is a great starting point.
Comet currently have the arcade version of the xbox 360 in a bundle with Viva Pinata, Forza 2 and PGR 4 for just £159.99 this also comes with an additional wireless controller. The package comes complete with the following:
Xbox 360 arcade console
256MB memory card
Wireless controller x2
Project Gotham Racing 4 (PGR 4)
Xbox live arcade (5 games including pacman)
All in all this really is a great bundle, taking into account the extra games and controller you are getting the console for around £100 which is the cheapest price to date.