You are much better off posting this in the other sections of AVForums.
If by cheap you mean sub £200 , you will probably be looking at an all in 1 DVD system. Plenty available, just make sure that they have an optical input so that you can connect it to your Xbox.
The all in one systems with DVD players have a built in Dolby Decoder, and an AMP. Think of it as getting the DVD player free.
If you try to set it up as seperates without the DVD player you will be paying a lot... unless you get a dedicated games "surround system" for £80.. which are generally a lot poorer quality than the DVD systems for similar value.