4 is what I have on most machines. It seems to be the bang for buck for business machines. If are running programs for photo editing, videos, large spread sheets and the like you can’t get to much. I settle for 16GB.
I do mostly CAD/CAM work which is not problematic when I am working with the simple layout designs. The greatest drain appears to be when I first start working on the final stages and do a lot of rotating models in three dimensions, or stress computations and mechanical simulations.
Even running with 2GB the machine seems to be struggling a bit, but memory is not all that inexpensive yet and so far I have simply tried to be patient and accept the occasional sluggishness. Thus the current musings on whether or not to upgrade the RAM on the main machines.
4GB isn’t enough. 8GB minimum. I spent $100 for 16gb and never get slowdowns at all. You have Excel/Word/Access, Email, and a browser open and you are sitting at near 6GB used. Browsers and email are real hogs.