MATLAB Gaussian Noise Generator, variance and power density

Hi,

I'm trying to use the "Gaussian Noise Generator" under
Simulink 6.3(R14SP3). There are something I dont understand.

1. Is there any mathematic formula to calculate the "Noise
Power Density" from the parameters "mean value(I set this to
zero)", "variance", and "sample time"?

2. What's the sample rate mean? Should it be the same with
the sample rate of signal, or the sample rate of the noise
generate decide only the bandwidth of the noise? If latter,
why dont use bandwidth instead?

Thanks for your answer! ^_^
Cheng