a) SAN -- how do I have failover
b) DAS -- two servers attached to two DAS for failover
Each node must be on two dedicated private networks for cluster heartbeats. Each node must also have one port (or more) on the public network so that database clients can reach them. The nodes must also be attached to shared storage over two ports. Ideally, the storage will be mirrored between a pair of disk arrays, so that there is no single point of failure.
The final piece you'll need is some sort of cluster management software. You did not specify the OS that your applications run on, but in general, you'll be well-served by using Veritas Cluster Server (recently acquired by Symantec) or the clustering software sold by your platform vendor.
This was first published in July 2005