I have the z-640s and right after I bought them I was very disappointed. The box says that you can enjoy 5.1 surround sound from DVDs and video game consoles. This is not true. These speakers are not for a home theater, I learned, but for the pc. The three 1/8 inch cables are for analog surround sound. The only way to get 5.1 surround sound of the the xbox by means of an optical toslink cable. An optical cable brings digital surround sound. So, to get 5.1 surround sound out of the z-640s, you will need to convert the digital output form the xbox into analog for the z-640s. To do this, I bought the Creative Labs
Extigy The z-640s put out an amazing sound- it's makes it sound even better since they only cost $60-80! You can spend around 120 to get the z-640s and the extigy, or buy the z-680s for 250. For a bedroom, I would definetely recommend the z-640s with the extigy. For a home theater system, howver, the z-680s would be better. Its nice to have all the optical inputs built in. The sound quality does not go drastically down as the digital sound is converted into analog. The three plugs are three stereo plugs- one left/right front, another left/right back, and the last one center/sub. The z-640s do not have a decoder- they need the sound decoded for them. That's where the extigy came in handy. It's quite an awesome product- well, at least to fit my needs.
EDIT:
Here is the Tom's Hardware Guide for the Extigy