Do I need to buy an amplifier in addition, if I buy one of these home theater systems?

Home theater in a box comes complete and you need not to buy an amplifier. They are not very flexible for adding anything to them or for upgrading. I would recommend an Onkyo HT-S, it is a separate receiver and a set of speakers. You can add just about anything to it and can upgrade at a later date. All you need is to add a Panasonic blu ray disc player model BD-75 or BD-65 for about $100 and you have a good system. You can demo the system at Fry's. Hope this will help you out.