After using our new server(s) for some time now, I can say that the performance is much better than we actually hoped for. We can start a build on our virtual build machine and everything else like our email and Subversion servers are as fast as normal. And the 4GB RAM gives us enough capacity for more servers and development/tests machines in the future. The actual performance and required hardware obviously depends on the actual number of users you have and our two users are probably not very representative.
But based on my observations and tests, I suspect that such a server should easily handle all the common services like Subversion, E-Mail, File Server, Active Directory and web based applications for up to 10 users or more and therefore makes it a very economical solution for small ISVs. And besides the space and cost savings such virtual servers provide, it becomes ridiculous easy to backup entire servers just by copying or burning the virtual machine files.