kde4 is sluggish if you don't have a good video card(or properly set) and enough ram, it requires more that those 512 you have there(specially if you have a integrated video card stealing more than 32 mbs of ram) and if you try to use compiz or kde desktop effects
with that cpu and ram gnome, lxde, e17 and xfce are good candidates to replace kde4 on your system
on synaptic press the reload button, update system and then search for task-gnome, task-enlightenment, task- xfce and others
there is a section ¿called tasks containing other desktops like fluxbox, icewm and others
after the installation, all of those desktops will be added to the kdm menu, on the button session type the ones you install will be listed