Collectd is a really nice piece of software that I found in the Sunrise overlay some time ago (when I was practising there to become a Gentoo dev :o). It's a lightweight, modular background process for collecting statistics on the running system. Data sources and sinks are wildly configurable, and you can even write your own plugins in Perl, Python, or Java. Now we also have it in Gentoo as app-admin/collectd, and if you need a quick way to visualize the resulting bit flood, there is kde-misc/kcollectd as real-time data plotter. Enjoy!