Determining the noise floor of a signal in the frequency domain
You need to normalize your data based on the type of window you are using to obtain your frequency domain representation of the data.  The normalization differs depending on whether you are measuring a narrow band (your signal peak) or broadband (noise) signal.  Once you have the data properly normalized, the power of the narrow band signal can be read directly from the data.  The noise measurement must be estimated from the "noise floor" of the normalized frequency data.  Your noise power estimate will be 6dB less than the noise floor.  For a detailed discussion,

Go to this link: http://www.fhnw.ch/technik/ime/publikationen

Download the paper ""How to use the FFT for signal and noise simulations and measurements".