There is a 320GB hard drive and a 250GB hard drive available for the 360 S consoles. Both of those units are installed inside the console.
You can also go with a USB flash drive made by San Disk specifically for the Xbox 360. There are 16GB and 8GB options.
If you have anymore questions regarding storage for your Xbox 360, please use this forum ---> forums.xbox.com/.../7.aspx