Using GridFS - Should it be on a separate DB?
Even if you keep the GridFS storage in the same database as your other collections, you can still choose which collections to shard (or not) when you need to move to sharding. That said, if you have it in a separate database, you will be able to more easily move it to a separate cluster if you so choose -- so you could, for instance, have a 3 shard cluster for your "main" collections and a 5 shard cluster for GridFS (or any other configuration you choose).
As far as sharding GridFS collections, please see the MongoDB docs on choosing a shard key for GridFS. Commonly, people shard the chunks collection (which is where the file data itself is stored) on files_id so that all chunks for the same file reside on the same shard. Again, please see the documentation page for more detail.