I use a E740 PDA with either MS Street or Rand McNally software in the car since I use the same unit with AnywhereMap in the Pipers I fly.
The PDA fits in a cheap vent clip cell phone holder and unlike in the airplane I don't use the external/additional GPS receiver since there are no TFR's on the roads yet. ;-) and I don't mind a little detour in the Xfire.
Don't know if you have a PDA already in which case this may be a cheap option since you can get the software for about $30-50 and a PDA GPS for $90-150.
I don't remember if the PDA version of RM has turn by turn voice...but they all have touch screen. :-)