Heatsinks are required to maintain reliable temperature range.
If the computer is in its original factory configuration with only 2 modules installed, those modules will be in the inner slot of both top and bottom memory riser board. First step to adding more RAM is to take the module from the lower board and install it directly next to the module in the innermost slot on the upper board. All addional modules will be installed in pairs - next installed pair goes into the two inner slots on the lower board. The pair after that in the two outer slots of the top board. Last pair in the outer 2 slots of the lower board.