if you have a 3 and 1 configuration you are running in single channel mode giving you limited bandwidth, if you switch it so that you have either 3 sticks, 1 in each channel or 2 sticks in each channel you will have slightly better performance. The i7 memory controller will run in single, dual or triple channel mode depending on the memory configuration.
In general though, memory is rarely the bottleneck and triple channel vs dual channel shows limited performance increases, i wouldnt spend more money on ram as a quick swap to dual channel mode will mean that you lose almost no performance and your system will never need to use that much unless you plan to do video editing in which case it might use up to 12GB of ram.