6.5's would be a big step up, I hope you can find a way to make them fit, but have you also considered a small amp for the fronts? It can go a long way towards sounding much better, and not blowing the new set.
I'll explain. When you turn it up too far the amp inside of the head unit just stops at a certain point so instead of having a nice musical AC sine wave, you have clipping. Clipping is like the power just hits a ceiling. Here is a picture:
You can see that the AC sine wave turns to DC at the top and bottom of the wave. This heats up the coil like nothing else, because it greatly extends the total time the coil is exposed to the full output of the amp. With a larger amp, even if it is giving out more power, it is doing so without clipping, so that power is for less time, and more importantly the coil is in full motion the whole time, so it is cooled by moving air.
And now back to the other problem...