If you have a client that needs a SharePoint 2007 Farm, with for example one SharePoint front-end and a index server, Microsoft recommends that you install, in the index server, Search Server 2008.
But to integrate this two products, you need to install the Infrastructure Update (Microsoft SharePoint Team Blog) on each server on the farm.
To install Infrastructure Update, you should start on the Central Administration Server, then a message will appear saying that if you have more servers in the farm, you should install Infrastructure Update on them an then return to the Central Administration Server and continue with the installation.
In one of the my Infrastructure Update installations there was a strange problem with one of the servers and the installation was failing. After research and looking into SharePoint logs i discovered that it's necessary to have the Windows SharePoint Services Web Application start in all SharePoint Servers for the Infrastructure Update to be installed successfully.
At last if you have clients that only wants to use WSS 3.0 you should install Search Server 2008 Express, because it installs WSS 3.0 and also the Search Server index capabilities.
