While you can use any HDMI cable with the Xbox 360 Elite, or any of the latest Generation Xbox 360 which include HDMI, if you want to hook up your surround sound system via TOS-Link, or SPDIF, you need the Xbox 360 HDMI AV Cable.
This cable is shaped “funny” to allow you access to the HDMI connection and breaks out Analog and Digital Audio output. If your sound comes just from your TV or if you have a high end receiver likely you don’t need this cable.
Xbox 360 HDMI AV Cable
Buy new: $41.99 / Used from: $21.95
On certain TV’s which only have DVI in, you will need this cable, plus an HDMI to DVI adapter.
Eforcity HDMI-F to DVI-M Video Adaptor with Gold Contacts
Buy new: $0.57
The Adapter is 57 cents, and would also let you use a PC monitor with your Xbox so you can likely find 57 cents worth of good out of it anyway.