Partitioning and formatting - verify me please

I bought an external WD hard drive 500 GB and want to do the following with it:
1. install and boot Snow Leopard from external drive (I use Leopard on my Mac) -must be GUID partition and OSX ext formatted
2. share files created with Mac OSX to computers using Windows (must be MS DOS FAT?)
3. share files created with Windows PC to Mac OSX (must be MS DOS FAT?)

I observe I can only choose a partition scheme for the whole drive which I guess has to be GUID partition table. I dont know how many volumes to format is best to reach my goal: 2 partitions or 3 partitions ???