Power Users-How to find them

We have over 350 computers on our network. From the Plant Manager to the poor shlups that are working 7/24. We are in a manufacturing environment. We need to get new computers and I am trying to find out who should get the processing power the new Dell 280s provide. What is the best way to guage a heavy user from a light-weight? What type of programs/usage can better use CPU and Memory?