Linux directory layout according to the FHS:
/ (5GB because of packages installed to /opt)
/opt - addon software packages (used in Solaris, Novell/Suse products)
/root - root user's home directory
/sbin - system binary commands (administration)
/tmp (2GB) - temporary files
/home (10GB+) - home directories of other users
/etc - system specific config files
/bin - contains publically available binary commands
/lib - shared program libraries and kernel modules
/boot (500MB) - linux kernel & files used by boot loader (1st partition)
/dev - device files
/var (8GB) - variable data such as log files and spools
/proc - process and kernel information
/mnt - mount point for mounting filesystems temporarily
/usr (20GB) - additional programs, source code, documentation, X windows, etc
/sys - kernel hardware device information
/media - mount point for removable media...i.e. floppy
/srv - server provided content

swap (2x the amount of RAM, or best judgement) (2nd partition)
if possible span the swap across multiple disks

minimum partitions needed: / and swap

inside of /usr:
/usr/games - educational programs/games
/usr/include - C program header files
/usr/src - source code
/usr/lib - vast majority of shared libraries
/usr/share/man - man pages
/usr/share/doc - application documentation (readmes, etc)
/usr/share/doc/HOWTO/TEXT or /usr/share/doc/HOWTO/HTML - howtos (packages: howto-text-en, howto-html-en)
/usr/X11R6 - X windows
/usr/local - location for apps installed from source

inside of /var:
/var/spool/ - daemons' files in process, printer queues
/var/spool/cups - printer queues
/var/spool/mail - email server's files in process
/var/log - system logs
/var/run - currently running processes

/boot, /home, /usr, /var, /tmp should all be on seperate partitions

/etc, /bin, /sbin, /dev, /lib, all must be part of the / partition