Dell Precision T7400 (Late 2007) - Manual (Page 179)

Adding and Replacing Parts 179
Memory Installation (With Optional Memory Riser Cards)
NOTICE: The memory shroud holds the (optional) memory risers in place; its
thumbscrews must be sufficiently tight in order to secure the riser cards and to
avoid damage.
The four memory riser cards that you received with your computer are
connected together in sets of two. Do not attempt to disconnect the memory
riser cards from each other. Each memory riser card is numbered in the upper
left hand corner. These numbers indicate which DIMM slot on the system
board each riser card must be inserted into. Only the DIMM_1-4 slots (the
slots with the white securing clips) on the system board can be used with the
riser cards. The other slots (DIMM_5-8) must be left vacant.
You must remove the memory riser cards from the computer in order to
install memory on them. The memory slots on each riser card are numbered
DIMM_1-DIMM_4. Memory must be installed on these cards in the order
that they are numbered; that is, DIMM_1 on riser card 1 must be populated
before DIMM_1 is populated on any other card. DIMM_1 must be populated
on each card before DIMM_2 can be populated on riser card 1, and so on.
Memory must be installed in sets of four with one of the four on each riser
card.