Server Virtualization

I’m an IT director for a non-profit with 50-or-so employees. We have three servers in our headquarters, running XenServer 5.5 and Windows Server 2008.

My question is: what is the best way to virtualize these three servers? Currently they host the following VMs:

* Application Server (CRM, QuickBooks, etc)
* Database Server (SQL Server 2005)
* Domain Controller (Primary)
* Domain Controller (Secondary)
* Email Server (Exchange 2007)
* Help Desk Server (Spiceworks)
* File/Print Server (w/ DFS)
* Terminal Server

So what would you do differently? Would you suggest more or less servers (hardware)? Or would you virtualize only two servers and use the third simply for backups?

I could really use some outside opinion. Thanks!