Hello,
I don't know much about proxmox but i know a lot about monitoring. Generrally monitoring depends on the type of architecture. If you have devices supporting Netflow, this could be very handy to identify bottlenecks or missues. There are just a few good tools for netflow under a low budget, try solarwinds or Pandora FMS.
For SNMP monitoring, probably the most common case, most tools do a good job: cacti, zabbix, pandora fms or nagios. OpenNMS and Pandora FMS have the best management of Traps, and only a few manage v3 properly.
For a mixed scope on monitoring: server, apps and networking, you have less tools, we use Pandora FMS for that reason, can manage netflow, snmp, wmi (for remote server monitoring) and agent based monitoring for unix & windows server.
Some links:
http://pandorafms.com/Producto/pandora-servers/en
http://openms.org