Depends on your mindset really.
If you want simplicity, plug and play and are happy to be locked in then a NAS will probably do.
If you wish to buy now for something you can grow in to in the future but may be a bit more expensive to buy and run now then a 'server' of some kind may be the thing for you.
One area where you may need to be careful with NAS boxes is their network ability. Integrated solutions around various Realtek chipsets, for example, have had bad network speeds to the point where HD movies over 12GB would not play for me smoothly (D-Link DNS-323). You are also at the mercy of the 'ecosystem' around that unit for support or hacked add-ons. With a machine running an OS (Linux or Windows) on an Intel / AMD mainstream chipset you can more or less take any app off the shelf for that OS and it will run without needing to get source code and cross compiling.
The Win7 or WHS 2011 is a no brainer as the WHS is built on Win server 2008r2, so build for just this use, and is cheaper than Win7
The HP Microserver as bremen1874 mentioned is great for this sort of thing. Low power, can run WHS 2011 which in turn will run most Windows software (for torrents, NBZ, ftp etc).
You could alos try;
A Supermicro X7SPA-H (Atom 510 1.6Ghz dual core with HT, 6 sata, dual intel nic) board
Lian Li PC-Q25 (5 hotswap bays + either 2x3.5" drives or 2x2.5" drives and 1x3.5" drive, standard PSU)
A 300W 80+ certified PSU (or even a Pico PSU)
SO-Dimm ram - 4GB
Should be around 400 quid. Add another 40 quid and you could probably get a X7SPA-HF-D525 (Intel atom D525 1.8Ghz dual core with HT plus IPMI 2.0 so KVM over IP or control the server from bios onwards via a web browser or IPMI application).
Take a look for a 6 bay NAS box with dual NIC for around the same price. Add in WHS 2011 @ 45 quid and look for a NAS at around 450 quid that is compatible with most windows software, handles 6 drives, has a PCIe 4x slot (could add a SATA card for the other 3 drives).
If you need the server to transcode and stream to your TV / XBOX / PS3 etc rather than just sharing a network drive and the player then decoding the file then something a bit meatier may be in order like a Llano or i3 based machine.
RB