3 way components
 

Would a decent quality set with 6 1/2" woofers in the front, and midrange speakers in the back be decent sound wise? I also dont want to change the headunit. for example HSK 163 - Hertz 6.5" 3-Way Hi-Energy Component System

But i remember reading on here a long time ago about some type of processor that you can hookup that bypasses the HU.

absolutely not. You're separating your midranges, and placing them in the rear of the car. 99% of your vocals will be behind you.

Haha. Got ya. Not even an option.

When I was looking I found the ms8 again. If you are not getting an aftermarket headunit is it worth getting that processor? I will probably get herrz speakers and the back left corner wicked sub.

And another question (sorry so many) but is it feasible to have the amp in the front of the car behind or under the seat?

Yup. The MS8 is a fine piece.
as far as amp mounting options, it would have to be pretty small to fit under the seat, but behind the seat will give you more room. Amps can be mounted pretty much anywhere, (interior of car, please!) and the newer technology keeps them running much cooler, so not as much airspace is needed around them as years ago.