I became a CNA this past winter, and I love it. Yes, it's not for everyone, but if you really think that you want to "help" people, I don't know of another job that is as intimate and "helping" as becoming a CNA. Also, another side of it, when something goes really wrong, you can say, "This is why I am getting an education and not doing this for the rest of my life." It pays pretty well, and though it is harder work, I don't get the burn out that I used to get at old jobs, maybe because this job is more worthwhile to me.
As far as volunteering goes, I have done plenty over the years at the hospital, but it doesn't compare to being a CNA. Now that's just from what I've seen. If you were working at a free clinic or something, maybe you'd get more exposure, but I don't know for sure. Things I did as a volunteer were medical records, mail, gift shop, restocking rooms...the only patient time I got was passing out menus and asking patients what they wanted to eat. There is a difference in that and being responsible for someone's well being.
As far as, "knowing the system," I get plenty of exposure just by being around the doctors/nurses/administration.
Also, I know that in Minnesota, you can just take the exam if you want. I took the class, and since I work at a nursing home (i'll be working at a hospital pretty soon), I got my tuition from the class back.
Hope this helps.
All being said, no matter what, you do need to volunteer, as well as clinical experience, so the choice is really up to you, and how much time/effort you want to put into it.