ADC speed and CPU cycles

I'm investigating using the MCB4300 or MCB1850 for an ADC-centric purpose, but I'd like to know how much CPU is left over if sampling at the full 400kHz.
I'd seen a review of the FFT lib touting its speed, but if we sample at 400k, can it also do FFTs?
If not, can one board do ADC and another FFTs, with some easy/reasonable BUS interface?

Thanks