Recently had to use an older computer for my cards, running ubuntu 12.10 32bit with 2gb ram and cgminer 3.5.0, using the latest drivers from amd. I have a 6950 and a 6970 but only getting around 500 kh/s with both cards combined, compared to 900-1000 I had when running on a win 7 64bit 8gb ram computer (also 3.5.0).
I don't have any special settings, just the pool information and an intensity of 19, no hardware errors, GPU_MAX_ALLOC_PERCENT 100 and GPU_USE_SYNC_OBJECTS 1 has been set.