From the SusteIT Project, a tool to help you decide on what kind of client to use:

This is an Excel tool designed to help Education Institutions and businesses estimate the costs and carbon emissions of thick (PCs) versus thin clients over a given evaluation period. The tool is designed to be easy to use and contains a number of default assumptions which can be easily modified. Based on the input assumptions, the results are displayed in a worksheet within the tool.

This is an updated version of the tool (June 2009) which introduces a factor for air conditioning in computer suites in the thick client energy costs worksheet. The tool can be used freely, provided that the source is attributed, and that the whole spreadsheet, or its accompanying text, is not incorporated into other materials.

http://www.susteit.org.uk/files/category.php?catID=5