Please be aware, that IF your ceiling is covered with pre-1990 artex then the following is something you should be aware of
http://en.wikipedia.org/wiki/Artex
Also ask yourself the following
There are a few options to look at.
1: Remove ceiling completely. plasterboard and skim over. (messy job if you have old lath and plaster ceiling type).
2: Dab and board over , then skim over with plaster.
3: Skim over with plaster. providing peaks are not too big.
4: Hire a machine to grind it off. (extremely messy).
5: Put up a framework of battens and fit new plasterboard ceiling.
Take into account how much you want to spend, what is the room to be used for, can it be emptied completely, do you plan to stay in the house.
Personally, I plan to stay in my house, and I always want a job done properly. I would remove it completely and plasterboard a new ceiling.