Good, fast, cheap... pick any 2 of the 3.
If you want 20 drives, you are pretty much forced to go with 8-port SATA controller cards such as AOC SAT2-MV8 (or AOC-SASLP-MV8 if unRAID ever supports them). These will not give you the highest performance for parity checks, but will do fine for streaming playback you want. This gives you some flexibility on the mobo, since you will only need 4 SATA ports on the mobo, but make sure you have enough slots for the 2 SATA cards, plus whatever else you need.
Do NOT get the Norco 4020... get the newer 4220... the backplanes are turned 90 degrees and give you MUCH better airflow.
Lets look at pratical minimums. Without getting crazy and limiting your mobo choices significantly, the lowest idle wattage (measured at the wall socket) you can go is going to be about 35 Watts for the whole mobo, and using an 80+ certified PSU. Fans and the idle wattage for 20 drives, will add about 20 more Watts. Parity check will be about 250 Watts for the whole system. Boot surge will be about 350 to 400 Watts.
After the CPU, the Northbridge is the place that sucks Watts. Avoid mobos with fans on the NB, or reviews that warn of excessively hot NB. Trying to get the 35 Watt mobo will limit your choices to things like the TA690G that have the low-power Northbridge and you will need to get CPU frequency scaling working under Linux... or undervolt the CPU. If you go with more general mobos, add about 20 to 40 Watts to the idle power requirements. Now an extra 20 to 40 Watts ain't much, until you consider it is 7x20 -- which is an extra 175kWh to 350kWh a year.
If building a new system from scratch, I personally would use all 2-TB WD Green drives... their price point is almost to that of the 1TB, and you can delay that second controller card, and you have much more expansion time before you run out of slots.
Also remember, that you will need to replace drives ... 3 to 5 years is the expected lifetime, so in 4 years, you will be tossing out all the drives you buy today (1 or 2 TB drives) and replacing them with what is cost effective in 4 years, which will likely be 8TB drives or larger. So even if you will have 20 slots, you will have tremendous upgrade room when you replace older drives at the end of their lifespan.
Many people neglect to do this: Calculate your annual consumption of storage space, and project it forward. Then take into account that hard drive sizes double every 2 years.
