Office SharePoint Server 2007 search guidelines
Mike Taghizadeh has posted some recommended limits on search configuration with Office SharePoint Server 2007 Search. There is lots of good information here. I have yet to personally see a client even come close to some of these limits, so it makes me feel good that Msft has tested these numbers.
http://blogs.msdn.com/miketag/archive/2007/06/25/recommended-guidelines-for-search-boundaries.aspx