Vicent, your ram usage may be far less than the reported from the plasma applet.
This applet reports the cahche and buffer, which should alone take up alot of unused memory.
My netbook, running KDE 4.4, Chromiun and Gnome Network Manager, eats 227MB of memory, but by adding the cache it goes to 667MB.
You can accurately check this by using free:
or by opening kinfocenter and checking the memory section:
total used free shared buffers cached
Mem: 1001 667 333 0 117 322
-/+ buffers/cache: 227 773
Swap: 1035 0 1035
Also, the taskbar and some other components of the plasma shell are only transparent when compositing is enabled. This is because they are not rendered on the Plasma Canvas. They are rendered as windows by X11.