In response to Markus's comment - we have many clusters, ranging from 2-node A/A clusters to a 6-node N+1 (N Active / 1 Passive) cluster for our SQL instances, and they all work very well, and are stable, the biggest issues we have are hardware related, not clustering. And, if your machines are running at very high CPU loads, your servers are probably underpowered, or you have runaway processes.

In response to Schadenfreude-Mei's comments (1) to be part of a Windows cluster the hardware must be identicle, No, the hardware doesnt have to identical across the cluster, it just has to be certified (for Win 2003 or earlier), or pass validation for Win 2008. (2) disavantages with a/a are that it is not a high availability solutiion r ather a high performance solution A/A clusters are still High-Availability, just with a extra risk that the remaining node may not be able to handle the load without degraded performance; to be a High-Performance solution, it would require a Load-Balancing cluster, not a Failover cluster.

The biggest issue you should watch for is if all nodes except 1 failed, could the remaining node handle all the instances. Benchmark the load (CPU / memory / IO), if you can, to get a good indication of the resulting load.