I'm not going to give you exact numbers but try to be generous with /usr and /var .
On our freebsd boxes we will usually start with something like:
/ 5gb
/tmp 5gb
(swap) 8gb (or two times memory)
/var 15gb
/usr 15gb
you may use more mount points like /boot or something special
/home normally would be the rest of the drive but thats up to you.
You have big disks but I am not sure what you will be hosting and how many clients you will put on a box, but you will really want to have good room on /var and /usr and I DO recommend making them seperate mount points.
You dont mention where you will put your backups, if you are making them remote or moving them offline then you would need a /backup mount point to at least hold them locally. However if you will just be backing up to your local drives and keeping them there then you will want a extra hard drive, or skip the RAID and make the second drive a total /backup mount point.
Nothing more terrifying than waking up with a dead primary drive and realizing thats where you put your backups. Some people backup to the local drive and then copy the backups off the machine via ftp or rsync. You have several other choices, but keep in mind you must account for backups somewhere.
