Many folks are interested in guidance on hardware. In the Team Foundation forum, Brian Harry, Product Unit Manager of Team Foundation, posted the latest preliminary hardware requirements.
If you are going with a single processor box (or if you want to maximize performance), I would recommend changing the warehouse update interval to reduce the load on the server.
One server, less than 20 users.
Application and data tier server
single processor, 2.2 GHz
One server; 20 to 100 users.
dual processors, 2.2 GHz
Two servers; 100 to 250 users.
Application tier server
Data tier server
Two servers; 250 to 500 users.
quadruple processors, 2.2 GHz