If you do the FFT of you signal (using matlab or scilab, for instance: remember to use also fftshift) you will have a vector starting from -fs/2+0.5 to fs/2-0.5, that is -199999.5 to 199999.5 Hz in your case. The step will be so that there will be N points (8000 in your case). Then simply build the frequency vector as f=[-199999.5:400000/8000:199999.5];
The FFT will return a complex result; you need the modulus, that is abs(FFT)
To recover the amplitude you will have to multiply the FFT by 2/N, that is 1/4000 in your case.
If your signal is "s", then use plot(f,abs(fftshift(fft(s))/4000); you will see two peaks (one at a positive and another one to the same but negative frequency) for each of the frequencies that are belonging to your signal