Basically, all you need is one partition for each distro.
There are different ways to boot, I prefer having only one /boot-partition where I copy all kernels + initrd. I install grub once - and once only! - then just copy those 2 files and edit /boot/grub/grub.conf
Most distro's can also be installed via ftp (with fast internet connection) by only copying kernel+initrd to the /boot.
And then, I would suggest creating a few extra partitions - maybe you'll want to have separate partitions for home and possibly other stuff? I have one partition for /home, one for /usr/local (where I store stuff every user should have access to - movies, pictures, some applications...)
Some people prefer keeping the boot-stuff separate (that is, no /boot-partition) and use chainloading instead. That, too, is easy to configure.