This deal has ended.
Overstock is currently one of the best places to buy an Xbox 360. They have the the Xbox 360 Platinum with hard drive for $398.99. This 10% off coupon (there is also a 12% off coupon for new customers) cuts the price to $359 but requires a non-media or non-video game purchase as well (here are cheap ideas). Shipping is all of $1.40 and there is no sales tax (except UT, IN). They also have very good deals on accessories.