I used to use one that was super tiny (the cpu and ram bars were probably 1 or 2 pixels thick), I also used it with windows 98 (I think) so it must be pretty lightweight. I'll look through my download backups later tonight to find the name for you.
What I got now may intrest some people on this forum. It's the Logitech G15 keyboard
. It has an LCD screen that you can write applets for. One of the few applets it comes with is a CPU/RAM usage meter.
If you'd like to see othe apps written for the keyboard, or if you just want some more information; check you www.g15forums.com