Upgraded maps and graphics on the Z3 is part of it, there are several others including DVD bypass, bluetooth fixes, etc. Reading through the AVIC forums will point out a lot of fixes.
You don't have to buy anything to do the upgrade if you have average computer skills, but I would buy a spare drive and perform the upgrade on that.
http://www.avic411.com/forum/viewtopic.php?t=9446
One of the forum members, Garrett, sells ready to go Z3 drives - plug and play for about $125. Watch for the Ebay drives, kind of a crap shoot - biggest issue is non-automotive grade drives that crap out with temp or vibration. I bought a plain drive for about $90 and did it myself because I wanted to keep the music that is on it.
http://www.avic411.com/forum/viewtopic.php?t=16080
Check it out, there are lots of fixes that the Z1 ->Z2 -> Z3 gives.
-Dave