Don't do plain striping, as there is no redundancy involved. IOW, if one drive dies, *ALL* the data in the stripeset, across all the drives, is gone.
How many drive bays do you have? How big is the power supply? Do you have any USB flash drives?
Until booting off a ZFS pool is reliable and safe, I'd recomend putting / and /usr onto a UFS filesystem. Then put all your other filesystems (/home, /var, /tmp, /usr/local, /usr/src, /usr/obj, /usr/ports) onto ZFS filesystems.
If you have a pair of USB flash sticks, you can install onto one, and create a gmirror RAID1 array across the two of them. If not, use the 80 GB drive as the OS/swap drive. There are a couple of different how-tos out there for creating gmirror setups.
Use the 3x 500 GB drives to create a raidz1 vdev. Add that to the pool. That will give you 1 TB of drive space.
Then you can use the 1.5 TB drive for backups.
Or, you are really feeling wild

you can add the 1.5 TB drive to the pool to create a mirror for the raidz1 vdev.
# zpool create pool raidz1 disk1 disk2 disk3
# zpool attach pool disk4
# zpool status