Moving an existing site to Mongo DB
I assume you're going to use the mongoDB module. This doesn't move the ENTIRE database over, just parts of it, like cache, blocks, watchdog, etc.

This works well because mongo, or any "document" oriented database is not great at relations. So you can move the data that is not very relational into mongoDB easily. 

Mongo itself is cake. Easy to install, easy to understand. It's simply "JSON" data. If you don't know JSON, read up on it now. Already, many parts of Drupal's data are stored as JSON inside text fields in a regular SQL database such as MySQL.

I'd re-iterate that you probably don't want to move EVERYTHING over, like the nodes, taxonomy, etc. Those things are highly relational. Also, the mongoDB module doesn't support those for that reason. 

Installation of the module is pretty straight forward and happens in settings.php. See the mongo module readme file for more info.