I agree the number of GPU's will most likely affect the use of RAM. However I know people running 5 290x on same mobo, psu and config using only 4GB, where as mine wouldn't work with 1 card on 4 GB. The only thing i could come up with was the us of another OS. Looking into windows reveals that 2gb of memory is reserved for 1 GPU

Might it be an option to contact the dev of sgminer since this is being maintained and updated for GPU mining and is essientially the same as cgminer?