If you want FreeNAS, do you also want ZFS? ZFS can do any RAID (including RAID5 called RAID-Z and RAID6 called RAID-Z2) and is the most advanced single-user filesystem on the planet. Its not widely usable though; at the moment real kernel-level implementations of ZFS are available on OpenSolaris and FreeBSD - including derivatives like FreeNAS which is based on FreeBSD.
FreeNAS uses FreeBSD version 7 which contains ZFS version 6
FreeBSD version 8, however, contains ZFS version 13 - more features and more stable.
For a serious ZFS setup, you would be wanting to run FreeBSD or OpenSolaris. But FreeNAS is excellent to get started within minutes. Still, if you want to use the full power of ZFS you need to learn a bit about it and how it can improve on your storage needs.
If you want ZFS, i recommend:
- 64-bit CPU (ZFS works with 32-bit but slower and not very stable)
- 2GB+ RAM (4GB or higher recommended)
- never use PCI
- all disks on Chipset SATA controller and/or PCI-express add-on card.
Note that you mix drives on the chipset-powered SATA ports as well as addon PCI-express controller; adding them all into a big array like 8-disk RAID-Z where 6 disks are on chipset SATA and 2 are on PCI-express controller. Never use PCI for storage - especially when doing RAID.
You cannot expand a RAID-Z or RAID-Z2 array (RAID5 and RAID6). So you cannot add a disk to an existing array while keeping your data. However, you can create a second RAID-array and add it to the same storage pool.
How does that work? You start with a 4-disk RAID-Z (RAID5) array. Later you buy 4 more disks and add a second 4-disk RAID-Z array to the same storage pool. Now your storage capacity will double. Essentially, ZFS makes a RAID0 array out of two RAID5 arrays. So you can expand by adding new arrays to the same storage pool.
Please ask more about specific questions you may have. If you want to know more about ZFS i'll tell you everything. Just know that it requires a bit of time for you to analyse and process all the information you're getting; perhaps confusing your earlier idea of a simple storage setup. But ZFS has many advantages - i strongly recommend reading about it, on for example wikipedia.