If I have already installed (screwed in the cement board to the studs in my shower, do I need to remove the cement board to install a moisture barrier, or will I be okay? If I must redo everything, what type of moisture barrier do I need to buy and install?

If this is a shower or other wet area I certainly would install the barrier. Cement board is not water proof or water vapour impervious. It is water resistant and makes a great tile based on top of a vapor barrier. I'd use at least a 6 mil visqueen

My answer is probably too late for Mitch, but it may help others.

I would leave the board and install/apply a surface membrane instead. It's a much better method of controlling vapor and at the same time you will be waterproofing the surface. If done correctly along with the proper shower floor waterproofing, you'll have a shower that will work as intended and look great for a long time to come.

I prefer the Kerdi membrane and Kerdi drain methods for showers. The way all showers should be built.

Jaz