scrypt-n seems way more touchy than scrypt about ram usage, at least with current beta miner stage. I had countless -4 errors / Gpu disable on my 7950 card regardless of parameters as long as I wanted to run it on a seperate command line than the r9 280 on the same machine with 4GB. Then I gave up and just used a single exeminer command to launch the 2 then adjusted gpu-engine/mem seperately with settings menu and it stopped bitching at me ^^
This is certainly not ideal since you can't run different thread numbers on each card that way, so the 7950 is a bit underused with 2 threads (instead of 1 + higher intensity) but it'll do...