Quote from http://www.omgubuntu.co.uk/2010/11/move-google-chrome-cache-to-ramdisk/:

gksu gedit /etc/fstab

Add the following line to the bottom of the text file that opens but BE CAREFUL – do not touch, edit or enter ANYTHING else:

tmpfs  /media/ramdisk  tmpfs   defaults,noatime,mode=1777 0 0