Well, in theory at least, you can fill a well defragged hard drive to its maximum capacity, but that will need to include the operating syste, page files and any temp files needed to run applications.
However, what you might notice as the drive fills up is that the system slows down. Defragging becomes a neccessity, as does regualr clearance of temp files, breowsing hsitory and any other ways you can save space.
For best performance, don't use more than 50% of the capacity of the drive. If you go over 60gb - 65gb on your hard drive snails may start overtaking you.