The Intel Core i5's memory controller is dual channel. To maintain dual channel operation the memory modules must be installed in pairs. What is the make and model of your 6GB of DDR3 memory modules.
I will only list Tier 1 motherboards and since SLI seems to be one of your requirements here's a list of motherboards with two PCIe 2.0 x16 slots:
ASUS P7P55D-E Pro
ASUS Maximus III Formula
ASUS SABERTOOTH 55i
GIGABYTE GA-P55A-UD4P
MSI P55-GD65 USB3