I use 20 GB and try to keep it at least 50% free. Your OS partion experiences a lot of file changes and you don't want it to become fragmented too quickly.
If you are a major gammer you might think about 30 GB as games like half life 2 are getting to be huge.
Definately keep your My Documents, Desktop, Favorites, Email Store foler to another drive. Move your pagefiles as well for smaller backups.
I like using Mozilla since I can store my entire profile on D: which means my email and web browser is always up to date even when I restore from a backup or install a new OS.