Hi Dan,
Unfortunately, I'm doubting that somebody will be able to provide you with exact specification but you will have to experiment according to your actual configuration and requirements.
One of the major points for your considerations should be your tested application. It is possible that test performance on the minimal h/w configuration (see requirements for TestExecute) will be acceptable if it is 'thin' web client running standalone on the test machine. But the C++/Delphi application of moderate size (~50MB) will require 1-1.5 GB of RAM. And I saw the .Net WPF application with very limited amount of actual business logic that was almost impossible to create tests for using TestComplete on Win7 64-bit machine with 2GB of RAM until the machine was upgraded to 4GB (but it is possible that tests execution under TestExecute might be good enough with 2 GB of RAM).
Anyway, I think that required RAM for OS and TestExecute will be not less than 512MB.
Also consider if the tested applications will run standalone or they will require some additional services (like DB servers, mail clients, different queues, etc.) to be executed on the same machines.
I don't think that any server can handle 40 actively running virtual machines -- this is one more point to consider. The hardware components should correspond one to another (otherwise, for example, slow disks can degrade performance of the server with powerful CPU and a lot of quick RAM).
Probably, you may try with whatever hardware you have at the moment, get initial experience with your application and your requirements and then try to estimate what and where in your test architecture should be upgraded.
