Question : Problem: DIMM 3A and B only support single rank memory

I'm trying to upgrade a Dell 2850 GII from 8 to 12GB RAM.  The MoBo allows up to 16GB and currently has 4-2GB sticks of PC2-3200R in slots 1 A&B and 2 A&B.  When I insert two more of the same memory, I get the message that slots 3 A&B only support single rank DIMMs, and I should remove the dual rank DIMMs.  If this memory works for the rest of the memory, what memory do I need to use in these last two slots?  

Answer : Problem: DIMM 3A and B only support single rank memory

What rank-ing is the memory?

From Dell
# If bank 2 contains dual-ranked memory modules, then bank 3 must be unpopulated.

# Dual-ranked memory modules are not supported in bank 3.

http://support.dell.com/support/edocs/systems/pe2850/en/it/t1393c60.htm#wp1172300