How comfortable are you with Linux?
While unRAID seems to be fairly easy to use, it wont fill all of your requirements (iTunes server, timemachine etc)
Linux has a very solid tool called mdadm - basically software RAID.
It's very powerful, will allow you to set up whatever raid array you want (raid0, 1, 5, 6, 1+0 etc), allow you to resize and grow arrays, and will also monitor the drives and report any faulty ones.
One of the biggest advantages of software raid is that you aren't tied to a particular piece of hardware. Hardware failure? - just grab the discs and throw them in another machine and you are good to go (with maybe a bit of trivial config).
There is also no performance hit over software vs hardware, particular for something like a media server.
I've set myself up an Ubuntu server (with no great knowledge of linux beforehand), running 5 x 500GB discs in Raid 5 - giving me fault tolerance for 1 disc and effectively 2TB of storage. It's been humming away nicely for a couple of years now. I'll soon start replacing some of the discs as budget allows, and expand my capacity.
Best thing is, being a stock standard Ubuntu server, I can also run whatever I want on it - web server, squeezebox server, mythtv PVR backend, torrents, etc.