What development boards have good RTOS support?
I've had lots of success with FreeRTOS, combine this with an ARM Cortex dev board such as one from Olimex (available from Farnell) - see This Page for a list of supported devices.  A Cortex M3 will run at 75MHz and deliver over 80MIPS. ARM code is dead effecient and some ARM Cortex devices include fixed point math functions.

If you want even more grunt, try a Beagleboard or RasberryPI.

As well as FreeRTOS, the latter will run Linux with Linux compiled with config_preempt_rt config option.