The T5500 in OP used triple-channel memory (and weirdly, I have it here as a 1P motherboard, not as a 2P, but the T5600 is 2P and I've no reason to suspect different), so if you had three DIMMs ...

An industry source has sent along information and images for a trio of new Dell Precision workstations ... sports up to 24GB DDR3 ECC memory. Just above that, we've got the T5500 (starts at ...