| "keep 2 smaller partitions ( 10 GB ) for the OS's"|
You can do that, but be aware that the amount of data on the OS partition will increase over time. Installing all new programs on one of the other partitions will help with that problem. I also move some of the other system folders (like My Documents, Favorites, email, etc) to another partition.
Keeping the OS partition small makes backing up and restoring the OS a piece of cake.