ISO on OM-D

Taking advice from this forum I've upgraded my G1 to the OM-D E-M5. Because of the lack of a suitable zoom the choice was either to go for a fast prime with the G1 or upgrade the camera to take advantage of performance at higher ISO.

The zoom I'm using is the Panasonic 14~45, which is f5.6 at 45mm. I was wondering whether an equivalent f stop can be calculated if I compare f5.6 at 100 ISO with f5.6 at 3200 ISO.

I'll probably be told that this isn't the right way of looking at things! But if it is, could someone help me with the calculation, please?