Absolute keV x-ray measurements have been performed with nanometer size rare gas clusters submitted to femtosecond IR laser pulses at intensities I <1017 W cm-2. Special care has been taken in the measurements, by controlling the different parameters, which play a role in laser-cluster dynamics, i.e., beam waist size, effective pulse length and energy profile of the laser, and the cluster size. In particular, the data obtained show evidence of low laser intensity threshold values for keV x-ray production, an optimum heating time when mapping the cluster dynamics by varying the pulse duration at constant laser energy and a saturation in the x-ray emission probability above a given cluster size.