Ars Technica has a pretty good article on buying laptops. http://arstechnica.com/gadgets/2012/08/the-ars-pc-laptop-buying-guide-back-to-school-edition/
I’m not sure they have enough information on the actual process of selecting the individual parts when you go to one of the online sites. HP, Dell and Lenovo will all allow you to build a customized laptop for many of their models.
And in the past, it was always cheaper to buy extra memory outside of the original manufacturer’s site. Does that still hold true?