There is a directory called 'planets' in the se4 directory structure under pictures--i.e. se4\Pictures\Planets. As this is a modified file, you may want to copy it to a mod directory such as se4\ModPack\Pictures\Planets.
I dunno the answer to the second question. My intuition says that these files just affect the creation of the map and so shouldn't affect an ongoing game. But I'm not going to back that answer up since I haven't personally tested it.
[This message has been edited by raynor (edited 05 April 2001).]