Also make sure it has all of the features you want...and on crutchfield they usualy have a list of ipods/iphones that will work with that certain HU under features and specs I belive..
Some things to look into... A head unit that bypasses the ipod DAC. Short explanation. your HU plays the music at better quality than your ipod would..
sub woofer preamp outputs are nice to have if you want a sub or plan to have one.
removable face plate if you are worried about theft.
variable color display to match your guage lights on the truck
its all about your wants and needs..