  Manipulation of the refractive index has been of growing interest lately. We
consider parameters and possibilities of enhancing the absolute-value limit of
the linear index in coherent atomic systems. Starting with a review of how
two-level transitions - without and with added coherence effects - do not
realistically allow for a significantly enhanced index at fixed atomic density,
we discuss a possible way around this using wave mixing. This confirms that the
only parameters, besides the medium optical depth/density, that can effectively
change the value of the attainable index are the frequencies of the involved
transitions.
