Korneliusz Jarzebski has done the numbers and produced a chart showing exactly how the RAM consumption of comparable KDE4 and KDE3 sessions measure up.
The result is a mindblowing 39% smaller memory footprint in KDE 4. This just goes to show, that it’s worth making large-scale changes to your desktop environment to get the fruit hanging on the higher branches. With more tweaks and optimization, at least we can hope that it will, if not much faster, slower than KDE 3.5 on the same specs.
Also check out a small and interesting post: KDE 4: like a dream on 256Mb/1Ghz/Intel