I am an audiophile---just home systems tho. Learnin the car setup slowly.
I just today had Infintiy Reference speakers put in. 6.5 in front, 4" in rear. The difference is wonderful. Not boomy, very responsive. All told, I'm out 150+/- 20 bucks for the speakers, and 80 for the install.
Next up is a 8" self-powered bazooka EL(from my old car). Then as sad as I am to do it, the factory HU is gonna get the heave-ho, in favor of a JVC Arsenal HU(it has a front MP3 input for my 5 gig player).
Then, a 4 channel amp and a EQ in the second half of the left-over DIN space. That outta be enough. Not the "boom-boom" by any means, but sharp and defined music.
Total in end...around 800-1000(the bazooka not figured in) total, install and materials. Hope that might help.