You might need a primer course in computer design but the short version is that some cards have a small amount of dedicated video ram and for more the design uses the computer's main memory. Not that this is a deep explanation but it is one.
Anyway we are getting the computer tomorrow, it's already paid for however we just haven't picked it up. The link before I posted the other day is not available anymore and that didn't have a very good desciption so http://www.circuitcity.com/ssm/HP-Pavilion-a6500f-Desktop-PC/sem/rpsm/oid/213659/rpem/ccd/productDetail.do#CustomerRatings a much better description I think.
The thing I don't understand is how can the video memory be partially shared? Wouldn't you have a graphics card or not have a graphics card so it be shared from the memory? I really need some help on this, I am really confused now.