Content on Mac Mini, XBMC/Plex streaming to Apple TVs/Roku?

I have an almost identical setup to the one you described. I use Plex Media Server along with a newsreader (PM me for apps to feed content to the newsreader) on a a dual core AMD Phenom II with 4GB of RAM and a 1TB drive running Ubuntu. This computer is in a closet with just a power and ethernet cable running to it. All of the apps that I use can be controlled through a web browser so there is no need to ever actually touch the computer.

I'm able to feed content to several devices at once without issue. I use a PS3 in the living room with ROKU boxes throughout the rest of the house. The PS3 can pull content from Plex even though it doesn't have a Plex app as well as allowing us to watch Bluray movies in the LR. Plex also has a nice app for iphones, ipads, and android devices too. I've found that running a PC as your direct frontend can get a bit flakey. The "Wife Approval Factor" went WAY up when I switched to dedicated media devices as the frontend and a media server just as a backend. The PS3 and Roku allows for us to have our streaming content as well as Netflix and Hulu on all TVs without the hassle of fighting with a mouse and keyboard connected to the TV (because with a PC as a frontend you can't ever get away with JUST a remote - something will break).

You seem concerned about the CPU. A 2.4Ghz Core2 Duo will let you transcode a 1080p stream or a couple of SD streams at once. My Dual core AMD Phenom II is plenty beefy.
Plex has a decent amount of documentation about processor speed here: http://wiki.plexapp.com/index.php/FAQ:_Plex_Media_Server_and_NAS
and here: http://wiki.plexapp.com/index.php/PlexNine_PMS_SysRequirements