You need to fit a 'split charge relay' wired to alternator & 2nd battery so that its charged when engine is running and you can use it, knowing that you can always start the car!
If you have a decent car stereo shop nearby, they can sort you out with one, or go mail order. All the simple wire instructions will come with it. You then need a voltage meter to give a guide to when battery is going flat after a few hours of stereo and xbox! You'll need a different type of battery to a normal car one, and I'm not sure on the price of these nowadays, but think they're about twice as expensive!
Personally, I've always preferred having rear speakers as it 'fills' the car with sound better, but a simple system can often sound the best. All my speakers are in the stock locations, with upgraded fronts (inc tweeters) and rears (inc tweeters) Try it with & without rears (seeing as you've got them), and see which you prefer.
Don't 4get the 12v power invertor for the xbox and screen!!!