It depends on where you are sitting. Regular stereo set up well and listening from the sweet spot already creates such a solid center image no center speaker is needed. Home theater is the way it is because it is intended for a large audience. Without the center channel anyone sitting off axis is going to hear dialog coming from whatever side they are on instead of from the screen. That's why its there. Well that, and to be able to sell more stuff. Stuff you won't need unless you can't sit between the speakers.
So anyway that being the case then the answer is beyond a few general tips like erik already gave nobody knows. You yourself can only try and see.
Your biggest sonic improvement by the way will be when you are ready to upgrade ditch the whole home theater schtick and upgrade to stereo. Two channel gear is so much better than HT just about anything you get will be a huge improvement.
So anyway that being the case then the answer is beyond a few general tips like erik already gave nobody knows. You yourself can only try and see.
Your biggest sonic improvement by the way will be when you are ready to upgrade ditch the whole home theater schtick and upgrade to stereo. Two channel gear is so much better than HT just about anything you get will be a huge improvement.