I would purchase a SatNav for the extra it costs. I purchased a Tomtom UK and Ireland and added the US map on line direct from the TomTom site. I'm sure there are other options but this one worked fine for me. One note of caution though. Do not rely soley on the gps, in fact I only use it as a back up toll and to find physical adresses such as at the hotel/campground we may be staying for the night. Good old paper maps should be your main source of navigation and planning as you can the see clearly whats around you and go where you want to go. A Sat Nav can [and does] add many miles to a journey while only saving a few minutes of your time. This is done by keeping you on Interstates and those few minutes could cost you in fuel and cheat you out of some great sight seeing opportunitys, so know your surroundings.

There is no single best car hire company as they all offer much the same and are competing to relieve you of your 'hard earnt'. I would certainly shop around on line and compare costs for your travel dates and make sure you check out the one way drop off fees and if you are under 25 years of age, the young drivers fee. If you are from Europe then you will almost certainly find cheaper rates booking in advance with a European consolidator, we recently got a great deal with rentalcars.com, but like I say, shop around and make sure it includes everything you need.