Arduino selection for RTOS I am brand new to Arduino and am having a difficult time understanding the "lay of the land". I'm looking for the beefiest, fastest, most compute-centric Arduino available to the market that will run embox.

As I understand it, there is really only the Uno and the Mega, both of which are 32-bit. However I also believe that 64-bit variations/designs exist and was wondering if anyone has actually implemented Arduino Uno/Mega as a 64-bit MCU. I also see there is the Arduino Due, but not sure how that fairs (beefiness-wise) with Uno/Mega.

So to summarize: What is the beefiest Arduino one can find, is it 32- or 64-bit, and will it run an embedded RTOS like embox?
While I'm not experienced with running an RTOS on it, I'd highly recommend Teensy 3.1 as the "beefiest, fastest, most compute-centric Arduino". FreeRTOS and ChibiOS both support Teensy.