Microsoft CRM 4.0's Data Migration Manager is a great tool for straightforward migrations. Just a quick tip though - if you are trying to re-use a map that you've used before (for example, if you added transformations to the map, and you had to break your source file into smaller files since the DMM only processes files smaller than 32MB) - make sure you either rename your second source file the same as the first, or export the data map and change the SourceEntityName part of the EntityMap tag.

For example, if you did a successful import of a file called customer.csv and wish to re-use the map for a second file called premiercustomers.csv, you would want to either rename your CSV file so it is customer.csv, or change the following line in the data map from:

<EntityMap TargetEntityName="account" SourceEntityName="customer">

to:

<EntityMap TargetEntityName="account" SourceEntityName="premiercustomers">

then save the map with a new name and import it into Data Migration Manager.