How can 20GB take most of your harddrive space? Small harddrives are old and slow harddrives; as their platter density is low too.
The best would be an SSD, like Intel X25-V 40GB which is very good for WoW; among the best really. More RAM helps too, but only after you've played the game a long time and never really reboot. If you reboot, all filecache in the RAM will be gone.
You on a laptop or desktop computer?