For safety reasons you should create a partition for the OS and apps and create another partition for data files (documents, music, movies, etc). If something goes wrong with your OS you can erase and reinstall without losing any important data since its being protected on a separate partition.
I found that 17gbs for XP is good while 22gbs for Vista is good, youll have enough room for a lot of programs without taking a big bite out of the data partition (games will not fit, I keep games on the data partition since they are frequently installed and uninstalled, and are so large). For OSX 20gbs is plenty for Leopard.