Network Architecture

My network has 250 nodes, made of 80/20 ratio macs and pcs. we are a growing advertising company that wants to improve our overall network performance, security, and remote access. As we grow, we deploy more PC to our new user.

We have a simple network,where all users are basically connected to a switch panel, and a ISP for internet. I am looking for ideas on how I can improve the network performance, and security.

We only have one router that interfaces with the ISP lines.