It all depends on if it will be strictly for music or both music and video. My ideal music only setup would comprise simply two towers that can accurately reproduce ~40Hz - 20kHz. Going down to 20Hz isn't utterly necessary, but a natural roll off to that point at several dB down is ok. Situate your listening position in a triangular fashion between the two towers. Picture an equilateral triangle. Voila! You now have a pseudo 5.1-7.1 system depending on your space and the reflectivity of your walls, floors, and ceiling. The "center" channel appears as a phantom channel. By aligning yourself between your speakers, this "phantom" channel exists. Reflections from the rear walls provide your "fill," or "surround."
Before anyone goes off and shoots this down, ask yourself this: When was the last time you went to a concert and the musicians were playing behind you? More speakers or larger speakers doesn't mean better sound. Neither does all of the other gadgetry that you insert between a cd player and the final amplification stage. He who equalizes least, equalizes best. Noisegates are something to look for as well.