Ubuntu 12.04 Multimedia Streaming Server

Budget : 350$ not counting HDDs but I would prefer to go with cheaper options (AMD) if possible.

Location : Continental US

Alright, so I'm working on a new headless media server build and I'm at a bit of an impasse when it comes to choosing the motherboard and CPU, as I'm not sure of the exact hardware requirements of what I want to do. My build as of now is...

APEX PC-389-C Black Steel ATX Mid Tower with room for 9 expansion bays
60GB SSD for the operating system disk

The case I plan on filling with (to start) two 2-3TB drives running under LinuxSoftraid in raid 01. I plan on adding 2 disks at a time when I expand with multiple raid 01 arrays.

My questions are as follows...

1) I want this to be a multimedia server that can stream to all the computers/tvs/settop boxes/consoles in the house, with often times 2-3 streams running at once. What kind of cpu processing power/ram do I need for this ? Clients will be either streaming via PLEX or AFP. I am looking at AMD processors ranging from $45 all the way up to $90-100. I'm not sure if a $90 processor would be total overkill, or not enough? Or if I should just bite the bullet and get an intel i3 for $120.

2) What kind of HDs should I be buying ? Will WD Green be fast enough to handle constant streaming ? Or should I go for something more powerful like a WD Black or Red?

3) Will I need HDD coolers like this ? These would drastically reduce available space but I'd much rather only have room for 5 drives and more stability then a system that will burn up. (4x drives with a cpu cooler, and the ssd which doesn't need any cooling...)

http://www.amazon.com/Thermaltake-TMG-HD1-HDD-Cooler/dp...

4) What kind of motherboards should I be looking at ? Is there any companies that list 'linux certified' boards or something similar ? I've read widely different accounts from people saying the motherboard doesn't matter at all to you need to check the chipset to make sure you get the right NIC. I'd prefer to avoid scouring hundreds of motherboard specification pages for chipset and NIC compatibility, but if that's what it takes....

Some programs I am going to be running....
PLEX linked up to the Raid Backup
Avahi & Samba for file transfer compatibility across operating systems
WebMin for basic admin, raid setup, etc
OpenSSH (not exposed to the net)
also possibly LAMP (not exposed, just for playing around with webdev locally) if there is enough available resources to run it.

I am looking to complete this system in the next 2-4 weeks or so, depending on when my tax refund comes.  

Originally this was actually planned to be a FreeNas server, but after hearing stories of poor performance, people having trouble with ZFS restoration, and freeBSD hardware incompatibilities I thought Linux Server 12.04 would be the best choice. Any advice would be much appreciated! I've spent weeks researching this build and it would be good to hear back from some experts.