There are a lot of very interesting and compelling solutions out there, and
PRTG is certainly one of the better known and more respected.
You can run MRTG/RRD/14all.cgi with 1 minute intervals. I'm doing that for a few interfaces and SNMP OIDs (CPU Utilization) but when you have 600+ switches to monitor and some 6,300 interfaces to graph you really can't be polling them all every 60 seconds.
I also played with
Nagios a few years back when it was called NetSaint and was very impressed. Since that time there have been a lot of start-ups and emerging products.
SmokePing is another tool from Tobias Oetiker, author of
MRTG, that might also be worth a look if your interested in graphing your network's latency.
Zenoss is also another option for the larger network management type of solution.
I'm going to stop now because the list is nearly endless... I'll let a few of the other folks chime in with their favorites.