NAS Drives

I would probably want to stream the audio and video to perhaps my Apple TV box (This part I am unfamiliar about and am reading all I can).
For redundancy I thought a 4 drive box would be better even if I populate only 2 drives to begin with. Has anyone any thoughts about specifying the required hardware?
I'm not sure I can get away with just 2 drives as I will probably want to put them in a RAID 1 Mirroring configuration and using TimeMachine as a backup programme while also having other partitions/drives as storage for both PC and Mac.
Ideally both computers could access all areas but I realise this may be difficult and would live with the PC backing up to a separate "area".
Any advice would be appreciated.