I just recently purchased a new MightyBoard (Rev G) for my Replicator that requires a new power supply.
The power supply for the Rev E was a 24 V/9.2 A which was necessary for the dual extruders and heated bed. I know I need a 24 V power supply, but should I be concerned about the amperage? What will a higher (or lower) rating affect on my machine?
I suggest looking at the maximum amperage draw for all components that could be on at one time, and then find a power supply that can supply at least 20% more current. You would never want to get a supply rated for lower current than your max draw, because then it will affect the torque or your motors, or the temperature to which your heaters can get.
Think of it like this: An outlet at home may be rated at 115V/20A. Your blender is not going to draw the full 20A; having a little extra never hurts. But if you try to run a large appliance (dryer, hot tub, etc.) on a smaller amp circuit, the breaker would blow because you are trying to draw more than it can supply.
The point is, get something rated for higher than you need, within reason. It will draw only what it needs.
Pro tip: Make sure to set the current limits on your stepper motor drivers for maximum performance!
A short, figurative answer from the electronical point of view:
A power supply (an an analogy you can view it as a water pump) as used by 3D printers is usually supplying a fixed voltage (a constant pressure going into your pipe system), in your case 24V.
The given amperage/current (the amount of water that actually flows) that is actually utilized at a given point in time is determined by whatever you hook up to your power supply (the system of tubes or pipes - imagine a valve like your tap/faucet). Now the amperage rating of your power supply gives how much current you can run through your electrical system (as is the flow of your tap/faucet by the pump if you had a limitless big, imigainative one).
The power that your supply can deliver is the product of voltage and amperage: P(power) = U(voltage)*I(current).
What you need to make sure is that a) you deliver the correct voltage, because this is what your circuitry needs to be specifically designed for (image the pressure of your pump being too low or to high, - you either won't recieve any water in the second floor, or your tubing can't stand the pressure) and b) that you can supply at least the needed power (otherwise you get a problem once you open up all the taps/faucets in your house, because they don't supply as much water as demanded). If your power supply can give more current, that's fine, it might just not be used. And as mentioned by Thetravellingfool already, keep a certain plus for losses and as a reserve, because no pump likes to run constantly at it's limits either ;)
