That's a difficult question - my preference would be 15gb for the OS and the rest split into three of equal size.
Keep one for your music
One for your documents, emails, everything you want to store and keep.
The last can be used for storing images of the OS when it is installed cleanly - say create an image with Norton Ghost.
Some say that there is no need to partition but by doing the above you have a fairly clean system that will be easy to reinstall if necessary.
An external hard disk is the next thing that you might consider.