File / Media Server Running FreeNAS

I am looking to build an inexpensive fileserver that will run FreeNAS ( http://www.freenas.org/ ) primarily to serve media files to my HTPC. I intend to use the ZFS file system. I have an old tower case that I intend to reuse but I will need to buy a power supply (and case fans if required). FreeNAS has a page on hardware requirements ( http://doc.freenas.org/index.php/Hardware_Requirements ) that may prove useful.

My intention is to eventually run 5 or 6 SATA hard disks in a RAID-Z or RAID-Z2 (ZFS specific analogues of RAID 5 / RAID6 ) although initially I will be using fewer disks and expanding as my budget allows. Hardware RAID is not necessary and best avoided when using ZFS. Read performance from the RAID must be sufficient to stream 1080p content. Write performance is not critical.

The system will addidtionally require a high speed compact flash or USB device for the operating sytem ( http://doc.freenas.org/index.php/Hardware_Requirements#... ). For quick backups to portable disks I would like an external USB 3.0 or eSATA interface.

To summarise I believe I will require the following:

Power supply
Motherboard
Processor
Memory (likely 8GB)

and possibly:

Minimal graphics card (if there are no onboard graphics)
SATA controller (if there are insufficient ports on the motherboard)
Case fans

APPROXIMATE PURCHASE DATE: Some time in the next few months.

BUDGET RANGE: As cheap as possible < £400 without RAID drives.

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: FreeNAS fileserver

PARTS NOT REQUIRED: Hardware RAID, Keyboard, Mouse, Monitor, Speakers

PREFERRED WEBSITE(S) FOR PARTS: I have used dabs.com before but have no real preference between reputable suppliers. COUNTRY OF ORIGIN: United Kingdom

PARTS PREFERENCES: I have a strong preference for parts with high quality free software drivers. FreeNAS (FreeBSD based) is of primary importance with Linux drivers being a secondary concern (in case I later choose to repurpose the system or its components)

OVERCLOCKING: No

MONITOR RESOLUTION: N/A

ADDITIONAL COMMENTS: I would like to avoid installing additional cooling if possible.

Form Factor: Tower