Well, 600 Mhz is fine with nothing running - but unfortunately, gnome
system monitor is far from nothing, and gnome itself, while much lighter
and snappier than it was a few years ago, is still more cumbersome than
the truly lightweight desktop environments which I think would be better
for your system.

If you were starting from scratch, that would be one thing, but since
you already have an existing install, you can just add the lubuntu and
xubuntu desktop environments via synaptic and see how each of them
behave on your system.

I'm adding the xubuntu desktop to my 10.04 box right now to see how it goes.