How many watts? AMD FreeBSD NAS w/ 8-12 hard drives

What sort of a PSU (ATX) would I need for the following new build. It is a system that will be running as a NAS server probably using ZFS file system and FreeBSD or FreeNAS.

AMD Athlon II x2 chip w/ 45w TDP
Asus ATX Mobo w/ 2 PCI-e slots (will fill one or two of them with SATA controllers)

I want the power capacity for 8-12 x 3.5 inch HDDs (probably 5400RPM)

No optical drive, no graphics card, nothing fancy (except a handful of case fans including one 220mm fan).

How many watts do I need?

HDDs use both the 5V and 12V rails from the PSU, right?

How many amps will I need on the 5V side and how many on the 12V side for this setup?

It needs to be energy efficient since this server will be running 24/7.

Here's one I am considering - SeaSonic SS-300ET (300W, 80+ Bronze):
http://www.newegg.com/Product/Product.aspx?Item=N82E168...