Ok, I have a dual quad core 2.6 mac pro. I added 8GB for a total of 10GB. So... I am using After Effects and the render takes all 8 cores. After effects uses maximun 3.5 GB and I have multiprocessing enable. The thing is that once I am rendering I can not touch the machine, everything slow down, even opening Safari is slow. The thing is that having 10GB is basically useless. In my experienece any mac can run perfect with 6GB, adding more is wasting money. After 10GB I do have more capacity of course but the system is not faster and everything is limited by the procesors capacity, period.