The main SQL server is on a SQL 2005 cluster. It is time for an upgrade to SQL 2008. This will be an interim measure until the hardware refresh the later half of 2012.
The main aim of the process is to minimise downtime. Install the pre-requisites, Windows Installer 4.5 etc on the passive nodes. You may need to move the active node around to accomplish this.
When performing the actual upgrade, the trick is to perform the upgrade on the passive node only. Move the active node around until all the passive nodes are upgraded. This has to be done locally, not via remote desktop connection. If the nodes are virtual machines, you will need to use the ESX client or HyperV manager to do this.
Microsoft as always provide detailed documentation but lacks a summary to give you a heads up whats happening.