Basically it is possible that KDE 4 effects (or Compiz) can show memory leaks in in grafic drivers, in that case the memory usage for Xorg will increase dramatically. Mostly older Nv drivers seemed to be affected by that so running KDE 4 on old machines with low memory was much slower than with KDE 3.5. I tested my own live systems now with a very simple test: show memory usage directly after boot using the integrated infobash tool (inside vbox with 980 mb ram) - rounded, basically it is changeing a bit with each run, but you see the tendency.
kde 3.5
32 bit: 129 mb
64 bit: 212 mb
kde 4.3 (backport for lenny):
32 bit: 179 mb
64 bit: 276 mb
kde 3.5
32 bit: 129 mb
64 bit: 212 mb
kde 4.3 (backport for lenny):
32 bit: 179 mb
64 bit: 276 mb
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