I know you asked about Google Earth, but I don't have it so I can't comment on it.
I went to our local soil conservation office and got topos and aerial photos of all of the areas I hunt. They are 1:24,000 scale topo maps and aerial photos so they both match up perfectly together.
For overlaying my Civil War maps on the computer I use Topozone. After I find out where the lines lay on Topozone topo maps I then coordinate them to the topo maps I got from the soil conservation office. I then draw these off onto the aerial photos that I go as well.
I get alot of CW maps that are very close to be almost 100% accurate. These are the surveyor's maps. I have found several communities that no longer exist. You can also get a general feel for the roads as well. Of course the scale on these CW maps don't show the exact crooks and turns in the roads, but you can still get a really good feel for it.
I have a few actual engineer(attached to a unit while in here on operations) maps that will show the exact crooks and turns of the roads with the plantations, houses and such labeled. Unfortunately these maps will not match up well enough for overlaying. I have been able to find most places on these engineer maps, just by using all of the reseources I have. Some places you have to travel the area to see what they might have been seeing for it to make sense.
I am in the process of recreating a whole plat map of my county during the CW. I have part of it done, but some places still elude me. Each time I do more research I come up with a few more small bits and pieces. Hopefully it will all come together for me. I have found schools, churches, houses, stores, etc. from the time period. Not to mention the camp sites for both CSA and Union forces. It's a very rewarding project.
The point is don't limit yourself to just Google Earth. There is a wealth of resources out there that will take you further.