Correct me if I'm wrong, but this #OptGreen project isn't talking specifically about Plasma, is it? They don't mention Plasma anywhere on the page they linked.
In any case, that's irrelevant, also, I don't doubt that KDE can't run at all under the specs you mentioned - that's not the issue. The question is, how much free/usable RAM do you actually have on that machine - let's say with no apps open first, and with then check again with Konsole + Dolphin + KWrite/Kate open? And for fun, fire up Konqueror as well and check again.