Just find a KML link online to topo info, such as the Geo communicator or NAtionl Forest sites. When you open them off the internet, your computer will prompt you to open them in google earth. I use google earth for both topo and aerial/ boundaries/ ownership/ water rights/ right of ways/ etc... all that info is on the net and Google earth will open them and transfer them to the 3D version for you. They work very slick.
Also, KMZ files can be downloaded and opened too.
When you do get them in google earth, make sure to move them to 'my places', away from the 'temporary places' section.