Listen to bascatman. The greenboard works great for damp areas.
depends on how much you want to spend and what your style is, you could glue on a new fiberglass surround, comes in pieces and just needs to be glued to the board OR tile the area, a lot of work, a lot more expense, but beautiful....
Keep the two bath/ shower walls the same, the other two or more walls you can use WR board (water resistant gyprock) witch you can get from any plaster board shop or bunnings. Good luck!
It all depends on your budget. If you can afford cement board throughout the bathroom, then by all means do it; it's designed for the moisture rich environment of bathroom. However, ordinary drywall works just as well. That's how I boarded the three bathrooms in my house.
Go ahead and finish the rest of the bathroom with hardibacker. You will never have a mold problem thats more than paint deep, or if you tile the walls, which you can now do cause you used hardibacker board mold will never get further than the grout. You made a good choice, any space thats prone to moisture calls for a cement type backing and hardibacker is so much easier to cut than the other products.
