System Activity now has a column "X11 Memory" as such:
This shows the amount of pixmap memory for a process. This value will be slightly lower that the value reported by Gnome System Monitor or xrestop because those programs take almost-random guesses at how much extra memory the processes use for fonts, pixmaps etc.
I have already created a bug reported against kmix which uses 1MB of memory for its dialog box which is almost always hidden. Feel free to report bugs against other such problems :-)
Originally I thought that this feature would not be possible because xrestop uses up a lot of CPU to gather these statistics. This is because it ends up doing an O(N^2) look up of processes, requiring an awful lot of round trips to the X server. I reduced this to an O(N) lookup and got the CPU usage down to about 30ms. Considering collecting and drawing everything else takes 60ms, this is still significant so this column is not shown by default.
Also, sorry everyone for breaking the build for those of you with weird X11 installations. cmake still gives me headaches. If you have problems compiling kdebase/workspace, because of ksysguard, with the latest svn version and after doing a make clean, please tell me.