Implementing HA

Modifying a system to be HA after the system is designed can be stupidly expensive, while designing an HA system in the first place is merely moderately expensive.

The question you need to answer is, how much availability do you need? To a large extent this is a business decision (i.e., do you have service-level agreements with your customers? Are you going to be sued if your system faults in the field? What's the availability number for your competition's equipment?); often just thinking about HA can lead to a system that's good enough.