Depends on the foundation. NC40 in one formula will not necessarily be the same in another. MAC's website has a Foundation Finder which you may find useful. http://www.maccosmetics.com/product/158/13055/Foundation-Finder/index.tmpl
I would not trust a match made by monitor. I would recommend you get matched at a MAC counter if you can, if not, tell us what other foundations shades are a match for you and we will guess!
I agree, you need to get matched at a MAC Counter. I have had a hard time with MAC foundations matching in the past, so I usually bring a foundation that is a perfect match for me, and match the foundations against that.