You have it right. When a head is ported, the contours of the intake and exhaust port are reshaped in order to improve flow. Some of the flow improvements come simply because the port has been enlarged.
The value of polishing the port is debatable for a street engine. Polishing smooths the surface of the ports and creates less friction as the air moves thru the ports. The downside to this is that less turbulent airflow allows fuel to drop out of the air/fuel mixture. Also, polishing is very labor intensive and costly for the minimal gain most people see. I suspect the real value of polishing comes into play at very high RPMs where street engines don't go.
To me, porting is one of those "black" arts that only a few people truly master. Sure, you can to a basic port job on a set of heads, but it won't come close to what a professional can do. That said, I would still do a little research and learn the basics like bowl blending, gasket matching, and general clean up. Those are worth a pretty good amount of power and you don't have to be an expert to do them. It just takes practice. I would find a set of junk heads to practice on so you can get comfortable with what you are doing before you work on the heads you are going to put on your engine.
When thinking about an engine, remember that it is basically an air pump. The more air that moves thru it, the more power it will make. A bigger cam helps an engine make more power because the bigger cam keeps the valves open longer, allowing more air to flow into the cylinders. However, this does not mean that the bigger cam will always make more power. Engine components have to be optimized as a unit as opposed to being choisen individually.