+1 to toby, for everything except the RAM
an uncompressed image of 20 MP with 10 layers is 800MB, add in a bit for undo data, and even the most unefficient program will handle that image in < 2gb of RAM
and all high grade image editing programs will compress on the fly, so that image above should take about 1gb max
from what ive seen CAD is vector, its CPU intensive, but not so RAM intensive. I dont see why a user would need more than 2gb of ram to run CAD extremely well (its proly changed since i used it like 5 years ago tho)