Just FYI - this is in the subwoofer thread, so you may not get the responses and assistance expected.
To address your question: I concur with lovinthehd, as it is good to remember about 90% of all important sound is coming from center stage. So the stronger and better that center channel - the more immersive the experience will be.
Like most - I have 2 towers for L & R... but the in-wall cabinet behind the screen doesn't have the height for another tower to sit in it... so currently, I have 2 decent sized bookshelf refurbished Infinity speakers as my center... I have 1 wired into the main receiver, then I have a pre-out feeding another receiver and the 2nd center speaker is wired to it so I can push them both and keep the internal receiver amps happy. I don't really have the budget, or shall I say, I don't want to allocate any more money to amplifying the current volume level - as it is a smaller dedicated HT room.
To conclude: My rule for center speakers is, the bigger the better - but budget rules all.