Intel Flex Memory technology dates back to 2004. It lets you use DRAM of different capacities to enable the exploitation of the motherboard’s multi-channel architecture whenever possible. Maybe you already have two 2GB DRAM modules and want to add a 4GB module. Generally, you would be running the 2x 2GB set in dual-channel mode (normally, slots 1/3 or 2/4). With flex mode, you can put the 4GB one in slot 1 and the two 2GB modules in slots 3/4 so that you have 4GB in each channel.