Any clustering application that you select, and there are many on the market, may introduce its own hardware requirements or recommendations.
With any high availability configuration that you choose, there are tradeoffs between financial and resiliency considerations. I don't normally plug my book in this forum, but this question is exactly the kind of thing that is covered in great detail in my book, "Blueprints for High Availability", published by John Wiley & Sons, and available at here. As for software to manage your cluster, it really depends on the OS you choose. Symantec's Veritas Cluster Server is an excellent choice that runs on several different platforms (though not at the same time). OS vendors also have clustering software for their own platforms, such as Sun, HP (Compaq), and Microsoft, and there are a bunch in the Linux space.
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