CompUSA is selling Xbox 360 Platinum Packages for $349.99 after $50 mail-in rebate, plus free shipping, until September 16. This offer is online only. They're also having a fire sale on the older games.
http://www.compusa.com/products/product_info.asp?&ref=cj&pfp=cj&product_code=334950&cm_ite=Xbox+360+Platinum+System&cm_cat=1755493
http://www.compusa.com/products/product_info.asp?&ref=cj&pfp=cj&product_code=334950&cm_ite=Xbox+360+Platinum+System&cm_cat=1755493