Go to crutchfield.com and enter the make, model and year of your vehicle; it will show you nearly all the options available. You can integrate to the existing head a few different ways or put in a new head unit.
I chose to replace the head unit completely and am happy I did so. My Alpine cost me ~$200 including tax, remote, dongle wiring harness, and fascia. It charges the iPod and gives a moderate amount of control from the head. Searching and selecting playlists isn't very easy, though. I prefer to unplug the iPod, navigate, then plug back in. Since the space is capable of a double DIN or 2 singles DINs, I was able to wedge in a Cobra CB.
A similar setup that lets you use the factory head can cost just about as much, so it was worth it to go straight to a better head unit.