I went for more of a high-end, but definitely less expensive system than those all-in-one systems sold by local and internet cart stores. I already had an overhead tray installed in the front of my 2005 Yamaha G21. I installed a relatively inexpensive car radio/CD player that has iPod and USB inputs in the center of that and added two 6" round speakers on either side. Crutchfield sells universal faceplate kits to make a nice install. I do still want to put a piece of black plastic above it to finish it off -- but it's not high on my priority list right now.
It's quite easy to wire up by just running the positive and negative wires down the driver's side post and into one of the 12v wires in the dash tray. I used one that switches off when the ignition key is off.
I then added these Yamaha indoor/outdoor speakers ([ame]http://www.amazon.com/Yamaha-NS-AW150W-2-Way-Outdoor-Speakers/dp/B0001VHARE/ref=sr_1_2?s=electronics&ie=UTF8&qid=1331223795&sr =1-2[/ame]) to the back posts behind each passenger's head. I also bolted them into the roof so anyone who tries to takes them out is going to get very frustrated-
I already had some of the components but for a decent, on-sale receiver from Best Buy and all 4 speakers, you're probably all in for under $200.