You should certainly bypass an inordinately slow system, but most performance differences among these PCs cannot be discerned with the naked eye. The aptitude of a corporate PC begins with its chassis design. The ideal corporate chassis is easily opened, and its major hardware is easily serviced to make upgrades and repairs quick and painless. An IT administrator should have the luxury of being able to remove a system's cover without a screwdriver and promptly access its CPU, main memory, disk drives, and expansion cards. In a tower case, you have lots of room and can reasonably expect all components to be readily accessible.
Machines using smaller desktop or even mini-tower cases require more thought.
Cables should be neatly tied. If servicing a machine takes less time, you can concentrate on other network administration tasks. That said, most vendors covered here do not offer an easily accessible chassis. Familiar makers including Compaq, Dell, Digital, HP, and NEC are generally the ones that offer such a chassis. The ultimate corporate box must also afford adequate room for expansion.