In a FWD transmission there's two shafts. The mainshaft has the clutch plate and six gears on it (counting reverse). The countershaft has the other half of the gears on it, and also drives the differential. The final drive is the gear ratio between the countershaft and the differential. So to get the overall ratio in a gear, you multiply the final drive by the gear ratio. And yes, changing the final drive has the same effect on every gear.
Written for RWD transmissions, but still informative:
http://www.howstuffworks.com/transmission.htm