It all depends on the load each of the servers will be placing on the hardware. If it is just to test connectivity with no or low user load then you should be able to get away with a Pentium Dual core. However if this is for a production envirement you need to look at the total CPU/Memory/Disk IOPS and network demand and work from there.
Sorry but there really is no one size fits all in virtual servers the same as physical hardware.
Remember to add the resources for the host O/S to your system requirments.