This very much depends on your needs. More memory and more processing power helps, just like faster harddrives and network-connections.
On the configuration side: it depends. Tweak the configuration to your specific needs - a very good start is this artikel http://kb.tableausoftware.com/articles/knowledgebase/optimizing-tableau-server-performance
And use extracts: querying a live datasource is most of the time slower than using a extract.