This deal has ended.
Overstock is again the best place to buy an Xbox 360. They have the the Xbox 360 Platinum with hard drive for $398.99. This 8% off coupon (there is also a 10% off coupon for new customers) cuts the price to $367 but requires a non-media or non-video game purchase as well (here are cheap ideas). Shipping starts at $1.40 and there is no sales tax (except UT, IN). They also have very good deals on accessories including this wireless controller ($49.99 at most stores) for $42.95 before coupons.