ok, so i know a few things about audio, but not alot...just wanted to get some opinions on quality to point me in the right direction...

i have a 1989 gmc sierra sle...im starting to put a system in it but im lost as to what i may need to do as far as keeping to sound quality high...i know that quality parts will help but heres the question..
the truck has mounting for 4x6 speakers in front and back. right now i have a set of polk 2 ways in the back and i have stock in front...cheap but decent powered head unit 60x4...the front speakers will be upgraded to the same polks as soon as i have the money..the speakers are 40 w rms and 120 peak...is it wise to run them through an amp, or is it overkill? the 4x6 speakers really wont produce much bass but i want to make sure i have every section well covered before moving on to the next (ie. subs)

also any experience with the c/k gm truck installs would be appreciated as well...thanks