I gotta say... I have a stereo which is 25W x 4 (excluding my sub). I had that high enough the other day that my ears were ringing afterward, with no distortion whatsoever, and the amp could've easily pushed more. So to me, any more than 25W per channel is not needed for regular driving (unless you want to use earplugs which seems like a waste).
HUGE Caveat: Since I have a separate subwoofer, I have a high-pass cutoff at either 100Hz or 125Hz (I forget). If you want to try to drive a lot of bass through your 6x9"s, then you might want more than 25W's per channel (but I doubt it).
Just my opinion as a user, not as a system designer or show participant.
I will point out that if your amp starts to distort before your speakers, it's possible to damage your speakers with the nasty signal (if you really plan to listen at high volumes a lot).
