We don't have for now the requierement needed for Lubuntu, because some components are still in developpement.
However, I done some testing on Karmic + LXDE/Lubuntu packages updated from Lucid + some adjustements, and a session start with 50-60 Mo, according to the taskmanager. So in your case, the memory could be a bit short to be confortable.
For the processor, I can't test if it's useable. But It will be a good candidate for testing Lubuntu Lucid, and I will be happy to have your feedback on it.