I wouldnt even bother with subs if youre not listening to music that doesnt require it. If your rear can accomodate a couple decent 6x9s, youre set to go. An inline amp can crisp up the sound and keep the head unit cooler, and if youre not running a sub, even a teeny 50w amp for just the rear deck can add the perfect boost. It depends on what you want to spend. I could put together a system for $200 with a satellite ready headunit, small amp, an four speaks--that would rival the factory setup in any high end car. Its mostly about putting it together right.
Of course, if youre not one to wheel and deal, you can certainly spend 5X more.