I've been using Torsmo on pdaxrom, it is nice and light and can draw to the root window. (I have had some problems using it with ROX' Pinboard)

Conky is a fork of the Torsmo system monitor (which is no longer being developed.) It is available for i386 in Debian, but not for ARM. Its page is:

Conky at Sourceforge

If someone could compile this for the Z, that would be great.