As with anything, it all depends on how much cash you are wiling to throw down and what you expect from it. I just did a budget sound system on my '04. I installed a pioneer 6800 deh headunit and pioneer two-way speakers all around. Some trucks don't come with rear speakers so you will have to run the wires yourself if you want them back there. Anyway, I got my speakers used for $75 and my headunit from ebay for $50. So for about $125 I installed everything myself and it sounds great. A hell alot better than the single cd player with cardboard speakers that my truck came with. I don't have an amp or a sub and it already rattles the hell out of my doors.
Also let us know what kind of music you like to listen to. Different brand speakers have different sounds. Kickers seem to have more mids and crisp highs, pioneers has good bass etc.
2004 4x4, ex-fleet vehicle. 4.0 v6 with 110 k, 4.10 gears.
Modifications: Whelan hidden strobes, Ion 174 alloys, yokohama geolander at 235s, billet grille, airbox mod, Dynomax exhaust. slight torsion crank. More to come!!