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 24v/9.2a which was necessary for the dual extruders and heated bed. I know I need a 24v 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!