IIRC Erin's review of the 650c showed it to be a pretty solid contender if you need a horizontal center. It is not without it's flaws, but anytime you lay a Speaker down horizontal you are going to have a battle of tradeoffs.
Of course the absolute best would be 3 750s across the front.
Are you planning on 750s at Surround, too? Or using the 350s?