It is true that just about any frame can be turned into a good fixie (except french frames), but i've seen enough good frames around for cheap that there's no reason to convert a murray. You might have to look a little harder for them, but they are there. Check you're local bike coop if you have one.

What makes no sense to me is when people spend $20 on a crap bike and then all of the sudden it becomes their beloved. They lavish time and money (generally more of the former) on it and are offended if anyone suggests that they might have more fun if they get a nicer bike. I guess my advice for anyone who is getting into bikes for the first time is this:
Other people know more than you. Not all of their (or my) advice will be good, but be open to the idea that there are things you could do better or change to make your experience more fun. And do lots of research on your own before you bug people. But just because you've done a lot of research, it doesn't mean you know anything.

This is good advice, I suppose, for dealing with anyone more experienced than you.