Check out www.combine360.com. This website seems to have the most information on Combine 360. They are also paired up with Under Armour, so there should be a lot of information through both companies.
No. You can trade in your xbox for whatever they will take and save money to combine until you get a 360, but you can not turn a regular xbox into a 360.
No you do not have to buy it.
Possible but pointless.
Under Armour Combine 360 is an athletic training program that encourages people to share their successes and failures. The following link will break down how it is scored: www.combine360.com
xbox 360
You can buy an xbox 360 used on internet sites such as Amazon, Craigslist, or eBay. If you prefer to see your xbox 360 before purchasing it, you can also buy used ones at GameStop.
You can buy a Xbox 360 s hard drive from the Xbox website.
Best buy, GameStop, etc.
360 is way better
$300
When Project Natal is released, yes, you will need an Xbox 360 to use Natal.
You can buy it for Xbox 360, PS3, or PC, but you can buy Modern Warfare 1 reflex edition for Wii (which has way worse graphics than the 360, PS3, and PC one) overall the PS3, and 360 are superior systems (360 fanboy!)