I highly recommend this little application. It gives an idea of what is going on that can be very important when you are on a dial-up modem and nothing seems to be happening.
However, I routinely connect my G4 powerbook at 28 to 31 kb/s here in Mexico but Menu meters gives me a very optimistic throughput of up to 6 kB/s with a 2 sec update interval. I also am confused about the meaning of the colors used in the memory usage pie chart (active, wired, inactive). However, when the pie is "full" I guess that I should not open more applications.