How do I justify upgrading to Windows Server 2008? [closed]

Question

We're just about to start a new greenfield project - it's a highly functional web application using ASP.NET MVC3, SQL Server etc. We're also going to be using Windows Workflow Foundation for the first time.

Our client only wants to use his existing Windows Server 2003 web servers. My main issue (other than it is 8 years old) is that we don't much experierence of WWF development, but understand that using AppFabric (Server 2008 only) will improve WWF development.

It's a significant cost to the client, as we need fail-over servers and a UAT environment as well.

Am I correct in my understanding, and what methodologies can I use to justify the cost of upgrading?