I'm looking to build a high performance Linux/Asterisk server which will be fairly CPU (transcoding) and LAN (high volume of small packets) intensive. I'm also not sure if I should go the Debian or Redhat route? I'm fairly new to the Linux/Open source space, but I have noticed that driver support can have a big impact on hardware performance in this space.
Supermicro's boards are all Linux/BSD compatible. If you Google "supermicro os compatibility", you will find charts for various models/series of motherboards. It will say whether or not it's compatible with certain distros.
Pretty much any distrobution will do. Ubuntu, Debian, Fedora, FreeBSD even, would all work just fine, as would dozens of other distros.