Well...don't forget about Swap!!

Actually I like to create a single large partition for /, and then a smaller (or maybe larger one depending on the function) for /var.

In freebsd...I typically do something like this:

Swap = 2x physical mem
/ = 256M
/tmp = 256M
/usr = 2G
/var = 1G

It really depends on what you are trying to do