Head unit and amp question

Ok, so I am planning on replacing my head unit and I am wondering if I should go ahead and replace the factory amps and wiring or just forget that and use them with an adapter. It's been so long since I have been into stereo that I'm not sure what to do.
What is everone's prefered replacement speaker now a days for our cars?

Also, has anyone put 8's in the rear deck before?

Ok, will do.
Now with that out of the way.

Is it better to stay with the 6x8's with seperate componants or go with 5.25's in the doors? I'm not really sure which to do here.