The only settings that gives more or less performance @ the GPU client are: priority - use low, and cpu/gpu usage - use 100%.
I don't know witch cards are you folding at the moment, but OC gives some extra help. Folding OC is not like gaming OC. Don't try to compare them. All I can say is patience. Use rivatuner, don't OC mem. Concentrate in shaders.
Change the values, run the console. When you start getting UNSTABLE_MACHINE. Go back. That's your limit.
Cheers