I agree with Curtis....
stick with the 170's! Why add different speakers to the mix? You will have the best possible sonic balance with identical speakers.
If I had not already owned a decent center speaker(B&W 600 series), I would have gone for the 340c because it is a "traditional" center channel(i.e. it is horizontal). Since you are restricted in the size of your center channel, take advantage of the situation.
Ed
* Sierra-2EX's W/V2 crossover upgrade
* (2) Rythmik F12's
* Parasound Halo P6
* Audio by Van Alstine DVA-M225 Monoblock Amps
* MiniDSP 2x4HD For Sub calibration
*World's Best Cables Canare 4S11 speaker cables