Hi PeteQ,
Do you have a usage profile for the users? What will the average user be doing on the web front end and how long is the average connection.
Have you tried the SharePoint Capacity Planning Tool?
http://www.microsoft.com/downloads/details.aspx?familyid=DBEE0227-D4F7-48F8-85F0-E71493B2FD87&displaylang=en
Regards
Nigel