unRAID is free for up to 3 drives; a trial of sorts. Support for up to 6 drives is $69, and support for up to 21 drives (and this number will most likely continue to grow over time) is $119. There's also a $10 off coupon code till the end of this month. The unRAID software pays for itself in the money you save on hardware.
No need to turn off anything w/ unRAID; it works great w/ all kinds of green drives. It's not a hardware RAID so those issues don't affect it. unRAID gives you the option to have your HDDs spin-down after a period of non-use, or to keep them spinning 24/7.
In unRAID data is stored on individual drives, not striped like in RAID5. So, if you want to access a particular file, only 1 HDD needs to spin-up for you to access that file (if you have the HDDs set to spin-down). It's a much greener system w/ the spin-down feature, but it adds a ~10sec delay when you first access your data when you get home as you wait for the HDD to spin-up. I have mine setup to spin-down after 3hrs of non-use. Turn it off for instant access 24/7.
Data is stored on individual drives like JBOD, but there is also a dedicated parity drive. In this way you can lose 1 drive w/o losing any data. If you lose another drive before you can rebuild you only lose the data on that 1 drive (instead of the entire array like in RAID5). Typically, drives don't just go completely belly-up, so since the data is only stored on 1 HDD you can remove that 1 HDD put in another PC, and try to recover the files. In RAID5 this is impossible...once the array goes down you'll be spending thousands for data recovery services if you want your data back.
As far as I know unRAID will only run off a flash drive. There is probably a way to run it off a HDD or SSD, but you would have to be pretty versed in Linux to figure it out. The unRAID forums are a great wealth of info, and on-going support.
http://www.lime-technology.com/forum/
The unRAID OS will not be sped up by an SSD as I think the whole thing resides in RAM during use anyway.
You also don't even need a monitor, keyboard, or mouse to run unRAID, but I would use them at first to make sure it's booting up correctly, etc. Once it's setup all you need is power and LAN. Most of your access to the system will be via a web interface from another PC in your network. From there you can manage everything, and of course you can access your files via Windows Explorer as if the files were local.
The CM 590 comes w/ (1) 4-in-3 adapter. CM makes other 4-in-3 cages that will work the same, but they look a little different and are aluminum instead of steel:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817993002
I managed to buy some of the cages that come w/ the 590 off someone on the unRAID forums, so that I have a matching set of 3.