You might be interested in "Mathematical Server Sizing" software, freely available on SourceForge. Here's a link.

http://sourceforge.net/search/?type_of_search=soft&type_of_search=soft&words=mathematical+server+sizing

Be sure and check out the documentation with this project. It describes some sample sizings. For a formal description of the math involved, see IEEE "Computer", July 2006 issue.

Good luck!