FreeNAS is good and as it is based upon FreeBSD it does support all the GEOM soft raid methods... however I would avoid using those and go for FreeNAS 0.7RC1, this comes with ZFS support, which is Sun's new Zettabyte filing system (look it up), but basically you can make expandable and mixed raid's with any level of disk parity redundancy you like along with hot spare / mirroring also. The bonus of ZFS is that all of your data is checksummed live in the filing system, so you can be aware of hdd problems/failures before they happen... + you can do ZFS snapshots which are "time machine" like...
But the only problem you may find is that zfs in FreeNAS is old, so it requires some kernel tweaking (especially if you only have 1gb ram), another consideration is that as you are using a single USB bus (in a notebook) that has a theoretical maximum of 580mbit or whatever... you are not going to get the best performance possible as this is your bottleneck... great for storing music and movies, bad for video editing / multiple-user NAS systems.
=) hope this helps.