Service Pack 4 for Sql Server 2000 introduced a bug. After it is installed, if Address Windowing Extentions (AWE) support is enabled, a single instance of SQL Server 2000 can only use a maximum of 50 percent of the physical memory that is on the server.
Microsoft has a hot fix for that problem here. I am happy to report that this hot fix is cluster-aware, based on my experience. You can run it on one node of your cluster, and the changes will be propagated to other node(s) on the cluster.
In my experience so far, all service packs and hot fixes, both for Sql Server 2000 and 2005, are cluster-aware.