Well, it seems like this is a pretty commonly asked question around here...
Really take your time on the job. I'd say that is more important than anything else. Using several thinner coats is also a good idea, as it results in a nicer finish in general, and also takes less time overall to dry. You dont necessarily need to use primer, unless you plan on sanding the metal parts of the case down to bare metal. This isn't really necessary, but some people like to do it anyway. You will definitely want to put at least 2-3 clear coats to protect the paint well. Additionally, something I would recommend, that most people seem to not do, is to wax the case after the clear coats are dry using car wax. I did this to my most recent case, and it really helps to bring out the shine, and also helps to protect from scratching.
Good luck on your painting!