mechanical load of an alternator
An alternators load on the engine will be however many watts of power it is producing, plus any inefficiency of the alternator itself.

This question: https://mechanics.stackexchange.com/questions/1536/how-much-power-hp-do-the-components-on-the-engine-belt-generally-use has an estimate of the power consumption of a 150 amp alternator at full load, being 7-8 horsepower. A 150 amp alternator is pretty big, and it's pretty rare that the alternator will be running at full blast.

You won't see very significant changes in fuel economy replacing the alternator, it generally doesn't draw enough power to be worthwhile to replace. Once your battery is recharged after starting up, the majority of your alternators power is probably going to be powering your car stereo or charging your phone, as the ignition and engine control systems only require a few handful of watts.

One thing that is interesting though, is some automobile manufacturers are considering eliminating the serpentine belt from their vehicles, and powering all auxiliary systems (AC, power steering, cooling pump, etc) by electric motors powered by the alternator. Apparently this can yield improved performance, or dramatically increase maintainability. 

edit: With all that said, these guys Remy International, a GM subsidiary, claim the opposite of my conclusion.