The one you want to concentrate the airflow on is the longer one to the left of the CPU socket with the row of capacitors. The heatsink below the socket is the southbridge which doesn't get hot.
edit: From what I've been able to gather from testing the CPU temperature in hwmonitor seems to be closely if not directly related to the VRM temperatures. Stability seems to go out the window past 78-80c so try to keep it at or below 75 in stress testing. You can take it further but I can't recommend it.