Axon Upgrades

I know that the internal parts of the axon have evolved over time as pc parts become available or die off. Im wondering when it would make sense to try and upgrade an early model, if a particular software release may start to be too much for them, or the sudden availability of more resolutions and higher res files might make them start to choke.

Is there a rule of thumb on when to upgrade them, or a list of what the possible upgrades are, i.e. what processors, video cards, motherboards, hard disks, etc are supported or when each type was used?