When to start sub-netting

Right now I have an NT 4.0 network with about 100 pc’s. We are an enginnering firm running ACAD 2000, 14 and Microstation on the workstations but job files are kept on servers, so we have a fair amount of network traffic. We also have a Exchange Server, Proxy Server and IIS. We have a Intranet site as well as our Internet site in house.
We connect with several client sites. We are in the process of growing fast and I do not want to get caught off guard here. With-in the next 2 months we will be adding an additional 25-50 workstations and within the next 2 years another 100 (total of approx. 200-250). Should I be considering breaking up my network into 2 subnets soon? Is there some type of guideline to go by.
Thanks