Output Voltage Noise and SNR

Hi,
Im trying to compute SNR for my circuit and have some problems with noise. I use INA326 IN-AMP and according to the datasheet :
h**p://focus.ti.com/lit/ds/symlink/ina327.pdf
(page 5) is the voltage noise at Gain=1 about 900nV/sq(Hz) and at G=100 about 35nV/sq(Hz).

1. First question and most important is WHY is the input referred output noise for small Gains so huge and for bigger G much smaller? Input noise is constant and shall be amplified by Gain. Why is this not the case ?

I compute SNR for 1kHz bandwidth , 1mV Input signal, Gain=5. So its on output Vnoise = (900nV * sq(1kHz))=28uV RMS, Vout = (5mV), SNR = 20log(5mV/28uV)=45dB.

2. Is this right how i compute SNR with RMS value of voltage noise ?

Thanx