That will work fine. For the most part during movie soundtracks the surrounds provide ambient sound to give you a feeling of space. The odd localized event like a gunshot or a car travelling across the soundstage can be handled by your current speakers. The nice thing about most dedicated surround speakers is that they are designed to be hard to localize while you are listening. When you have the money you can upgrade, but for now I would do exactly as you are planning.