Custom Demo Shopping Cart

I need a demo shopping cart system installed on our server. The shoping cart will they typical shopping cart with registration options for the users but integrated to a private payment gateway. We will provide the api to connect to our proprietary gateway which you will have to code too. The project will be hosted in a linux server. Delivery will be Sunday 21st. Award will be Feb 19th 2010. The final product also will have to have a clean mordern look and feel. You can use what ever open source shopping cart system, customized for our project integrated to our site. Hosting @ godaddy. Regards