A really good option in this field is IceWM. It has everything a typical
user needs out-of-the-box and is extremely lightweight (and themeable).
From my own experience I agree about that.
Still, the tricky part will be to choose other gui programs that are
still usable with the OP's hardware. For example, if they need a gui text
editor, nedit may be light enough for such a machine (that is, if one can
live without proper unicode support) and maybe xfe may still be a usable
gui file manager for them. The display command provides probably a usable
image viewer.
Web browsers will be especially tricky.
If dillo is good enough it will probably behave more or less smoothly.
Using firefox would very likely be not much fun.