Is it correct to use "wattage" and "amperage" instead of "power" and "current"?
Some concepts, like "power" can have multiple different associated units depending on the context. For instance, power can be Watts, Kilowatt-Hours, Joules, etc.  By using "wattage" in that situation you are being slightly more specific about the units used.

In general though, most times it's completely interchangeable, since the units and the concept are tied together - potential difference is always measured in volts, so "voltage" is perfectly acceptable.  Current is only measured in Amps, so "amperage" is perfectly acceptable, and you do actually see it around the place, especially for things like fuse ratings.  Resistance is resistance is resistance.  You could use Ohmage, but no one would, because it's resistance.  Of course, the Ohm is used for other things than just resistance, like impedance, so you'd use "resistance" and "impedance" to distinguish the two, whereas Ohmage wouldn't be able to distinguish them.