I think to remember there was a maxscript command to clear the memory
but I can´t find it in the help.
I need to attach several thousand CAD imported objects.. and max crashes
after ~100. This will be great fun 
Better to do it scripted for many objects in order to avoid crashes for attaching.
use disableSceneRedraw to stop redrawing
use undoOff() to turn off undos
and use gc() (garbage collect) to clear unneeded memory
Regards,
Thorsten
i had this same thing importing a revit file once.
The quickest and easiest thing to do is attach 20-30 objects at a time. Its a very tedious process but its the quickest way of doing it.
If you are attaching mesh, try to convert to Edit Poly and then attach. Edit Poly seems much more efficient at memory handling than Edit Mesh.
I used that trick a few years back when dealing with some very high poly trees from onyx.
Thank You for the Tips !!
the big one though is
freeSceneBitmaps()