Here are some thoughts on that. Some advantages of a 2nd server would be to balance the workload between servers for AFP and other services. Criteria to consider here would be the number and types of services you have running on your servers. Another consideration would be your network infrastructure. I believe a single OS X server is capable of handling 175 simultaneously logged in users. How efficiently it can do this depends upon the type of work being done and how big the files that are being moved around might be. If your switching at gigabit throughout the network, with gig nics in most of the workstations, it should be fine. However YMMV. Another minor consideration woud be security for staff and admins. Good passwords for staff here should handle that consideration in your environment. If your single OD server is efficiently handling the load which you are putting it under now, I would think that there would be no need for a 2nd OD server.