I have just bought an HP microserver, as it was very cheap for what it was, and does the job nicely.
Its more than just a NAS though. I was looking for a NAs, but found 4-bay NAS devices to be quite expensive. Even 2-bay NAS are quite expensive.
However, the HP Microserver (N40L or something) has 4 drive bays, including a 250 GB 3.5" HDD. But has a 5 1/4" bay too, so you can put a BR drive on it (for ripping media straight to it, or playing from media etc), or you can put another HDD in there (any size, 2.5" or 3.5"), or if you add this:
Scythe Kama Rack 5.25
You can add 2 2.5" SATA drives (so maybe one SSD for your OS / NAS software, and one for convenient swapping between PC's / Laptops). This frees up the 4 bays in the server for larger storage. In theory, you could have 4 x 4TB 3.5" HDD's, 1 x 2TB 2.5" and a 240 GB SSD, giving you 18TB of storage out of this little box. Should hold a little bit of media to keep you going.
I am running Windows 7 on mine just now, until I build my next PC, then I will run dedicated software (Freenas most likely, but possibly Windows Server 2011).
The best thing is that the basic machine costs £250 or there abouts, but you get £100 cash back. So only £150 initial outlay, on a device capable of 18+TB of storage (assuming you have the coin for the HDD's, 4TB HDD's are coming in at around £250 each, and 2TB 2.5" at over £160, so £1160 + for that amount of storage). However, 2 TB HDD's are cheap, so you can easily get 8 TB + in there for not much coin.