VMware Capacity Planner does this but it needs to be run for quiet some time to have a good dataset to run off. this is more for a virtualisation project where we are consolidating a datacenter
Personally if your only looking at a couple of servers run perfmon on them see what average cpu mem and disk they are using and then make a well educated guess as to what is needed.
Remember its easy to add resources to virtuals if needed.
Cheers