Gertrude,
In my experience it sort of depends upon what you are doing.
I do a lot of image processing, 2.5Ghz Quad core m/c was bought with 4, went to 8 and finally 16Gb of RAM; quantum leaps in terms of number chunking every time. I am aware of the requirement of scratch disk space; I run two 500Gb drives as a RAID 0 array for fast seek/write times; with a maximum of 200GB of data to give large scratch disk volumes.
MM.