I have 4GB of RAM and 512MB of RAM on an Nvidia 9800GT. I recently switched to LXDE simply because the entire KDE desktop lags graphically. I've tried turning on and off various KWin switches, and still KWin's FPS monitor often shows drops to > 24FPS when moving windows, etc. I've changed graphics drivers, Nouveau vs. proprietary, etc.
More importantly, I realized I use none of Plasma, and the KDE 4 series just has a certain feel to it that I don't like. Almost as if all of the widgets have too much margin around them, or something else that I can't quite put my finger on. Lastly, vertical panels / taskbars just don't work like I want them to.
Overall, I think they have the right idea. Plasma Active makes a lot of sense, and it's proving some of the logic in their strategy of generic reusable widgets so they can easily build environments to match devices, rather than one environment to fit them all. It just looks like it's going to take them quite some time to get there.