@heidrun It is a much better case to include as much system information so that the respondents can offer you opinion with more accuracy. Again what is your budget? What is your goal?
A centre speaker takes the speech load from the side speakers when playing movies, for instance. It adds focus to that part of the speech score, while other speakers are available to take the music score and special effects.
1. I am assuming that a Sub-Woofer may be a plausible inclusion. This will fill in the lower notes and give you the visceral feeling in lieu of a full 5.1/7.2 etc system. There are some excellent Subs out there
2. There are some excellent soundbar/sub combinations (Yamaha/Samsung) that will lift your roof off.
There are very cheap ($$$ wise) amplifier/speaker combinations that can be considered as a typical 5.1 surround layout.
Remember that a good surround sound system and sound as good as a stereo system
Without bags of gold to purchase 2 systems (1) for Stereo only and, (2) for star wars big effects, you are going to need to plan your equipment requirement carefully and moving through the second hand "bins" and forcing a bargain there. Good luck.
AMG