ADuC812 is a 8051 derivat for sure. You can use it for FFT but it depends on requirements that you have if the ADuC812 resources are sufficient.
I have already implemented a digital filter using PHILIPS 87C749 with builtin AD. I have used Goertzel algorithm for a single frequency detection in a real time. So the sampling frequency is 5200 Hz and I make a fixed point 24 bit arithmetic at that rate within a timer interrupt routine. Each instruction was optimized separately and all redundancy removed that I could meet the requirements. AD sampling and Goertzel computation in interrupt routine takes 70% of the CPU time. Goertzel algorithm is a second order IIR filter with a very high quality and low memory requirements (16 bytes of RAM was sufficient.) 87C849 is running at 16MHz.
This is my experience. Consider your problem and find out, if ADuC812 can solve it or not. If CPU power is not sufficient for complex matrix computation, maybe you should look for a faster 51 derivate (like DALLAS has, or even in the worst case take a DSP).