I have some experiences in decoding low freq. (up to 1 kHz) signals using PHILIPS 87C749 & 16MHz, but only for detecting tone burst on mains voltage used to transfer information. 8051 processors have not enough compute power for a real time application. (I was using IIF digital filter and 24 bit fixed point aritmetic, sampling rate was 5200 sps and sampling interval 60 ms. It takes 70% of CPU time to do the complex computing in an interrupt routine. I think that this is not enough for you. Youd better take a signal processor.
For IIF algorithms (for example Goertzel algorithm that I was using), visit www sites of Texas Instruments, Analog devices, Motorola semiconductors or maybe some others. If you want more information, please send me an email.

Hope this helps.