Good for you! Very awesome that you're seriously considering chasing your dream, especially if the opportunity is there for you. Programming is an interesting trade with many, many niches.
If you learn how to do game programming, you will have a great feel for high performance programming, which will translate into many other jobs if the game field doesn't have any opportunities. For whatever reason, I've fallen on the web app track and probably will never leave it, for better or worse. Regardless, the skill is extremely portable, and I'm certain I could program anything at this point, but I'd definitely lose my level of seniority (and pay).
Anyway, I think you should definitely ask as many questions as possible when you sit in on that class, but don't be overwhelmed by the topic matter. 2D programming is no cakewalk (though nothing compared to 3D) and far beyond an introductory level.
If you decide to pursue the career, be sure to take on a personal project on the side. For the first 6 months of college, the entire programming thing just didn't click for me. At the time I was really into "acquiring" movies, so I decided to write a little app that would let me track the stuff I had and the stuff I wanted. Doing that taught me MANY valuable lessons and really brought into perspective all of the concepts that the professors had been trying to drive home. It didn't make me an expert or anything, but it provided the platform I needed to grow.
I eventually started work during my second year of college and learned an exponential amount more. It was only a small start up, but I was responsible for a lot and thus had to internalize and learn a lot very quickly. This gave me an even greater platform to continue to learn and grow. Take every opportunity that comes at you to grow your skills because programming is all about applying your knowledge. Reading books and doing assignments is great, but it will never flex your mind in quite the way applying yourself to a project will.
The bottom line is this: if you want to be a great programmer/game designer and dedicate yourself wholly to it, you will be. If you want to learn more and actively pursue knowledge, you will learn more. The more you love this craft, the better you'll continue to get at it, guaranteed. Every single impediment you encounter is really just a challenge that will grow you.
Given the passion I've seen you exude, I have no doubt that you'll do great as game designer/programmer.
