Nas4Free and FreeNas are amazing. I have yet to decide for sure which one I will be using permanently . I have installed both and set up the system in RAIDZ-1 and both work very well and are easy to configure.
All the parts in this built except the HDDs and the CPU and memory were pulled out of retirement and given a second chance at life. I got the CPU from amazon used for 17.99.
The thing about WD green drives is the Intellipark factory setting is 8sec. That is bad for a NAS setup. To get the best performance this can be changed to match the RED drives 300sec or the blacks disabled. I disabled it.
The bottleneck is the network. It's all wired with gigabit speed parts and I get full 1 Gigabit transfers from pc to the NAS and back. with a second NIC in both my PC and the NAS I could possibly using link aggregation achieve 2 Gigabit speeds.