Network Monitoring Tools

I would like to solicit recommendations for network monitoring utilities.
We are attempting to monitor around a thousand nodes (and growing). We're
using SunNet Manager since we're primarily a Sun shop. The problem is
it's taking a full time programmer (me) to keep up with the changes in our
net. It doesn't seem like it ought to take that much effort. The
operations staff is competent and ought to be able to configure a
monitoring utility. Also, SunNet seems to be a cpu hog. At the rate it
uses up cpu, I'll need a giga-SPARC by next year.

So, the question of the day is what's in use and really works ? I'm not
enamored of cute GUI interfaces, particularly ones that try to draw my
network out before giveing any useful information. Among other problems,
running these over a less than wonderful connection to my hotel room is
painful. Something that scales well into multipe thousands of nodes would
be nice.

Thanks in advance.