Firstly, you could mail copies of everything to yourself, registered delivery, and leave it sealed when it arrives. You then have a date stamped, sealed copy of your artwork.
On the subject of cloned games, there is the issue of "Prior Art". Even if you were to copyright Pac Man because you realised nobody had ever officially done it, you could never sue because of the existing examples available before you copyrighted the idea. This is the case for many games, nobody really knows who came up with the idea first, and there will always be a case of "Prior Art" if a specific example was used to lay claim to the idea.
The wheel is officially invented by an Australian guy in the year 2000! He realised that nobody had ever patented the wheel, and did it himself.
Imagine how rich he would be if it wasn't for the rule of Prior Art!!! He could sue Ford, GM, Fiat.....