What you want is Serious Samurize.
Their home page appears to be down just now, so <a href="http://en.wikipedia.org/wiki/Samurize">here's the Wikipedia entry</a>. You can download it from lots of places, although their home page is a good place to start. Once, you know, it's available again.
It's so configurable that when you start getting into it you sort of get vertigo. But I've run it very happily for several years as a teeny box in the bottom right hand corner of my screen telling me instantaneous network throughput, CPU and memory load, etc.