How to Monitor Network in Medium-Sized Company?

I work at a medium sized company (100+ employees). An issue that has been cropping up is network performance, internet access in particular.

We have about 70 or more computers, a mix of Mac OS X and Windows XP & 7 machines. We have several servers (Exchange server, PC file servers, MS SQL, Blackberry, FTP, Mac server, etc). There are four main switches, a SonicWall firewall, and probably a couple routers in the server room with a dozen or so more scattered around the building.

The network structure has grown organically over a number of years; and, as far as I know, there really isn't a monitoring solution in place. When we experience network issues (slow connections, dropped packets, and so on), our general solution is to power cycle some hardware or go around to each employee and ask them if they are uploading/downloading any large files.

This is really inefficient and time consuming, and it does not allow us to monitor the network, tackling potential problems proactively. I would like to find a solution that would allow me to monitor network usage company-wide in real time, with detail going down to the individual computer, ideally.

Given the hodgepodge of equipment and operating systems, what would be the best way to set up some kind of monitoring solution? Hardware, software, restructuring our network architecture?

The first step is to monitor everything. I can suggest you to use Cacti or Zabbix to get SNMP information from your devices so you can find exactly who is using how much. After that you can even setup alerts (if you use Zabbix, or then install an extra tool like nagios) based on usage or problems.

After all that you can think in ways to segment your network, optimize, buy new stuff and etc.

"you can't control what you can't measure"

I recommend the time tested and proven combination of Nagios and MRTG. Nagios for monitoring and alerting and MRTG for longer term monitoring, which often shows trends that may otherwise go unnoticed. There are alternatives but I've found these two can do all the others can but are more easily set up and configured, although that might be just personal preference.