Yes Apple/Mac is known to be the one when it comes to multimedia editing and creating. Although Windows based machines are gaining ground in this market Macs are still the favorite when it comes to large professional organizations.
Apple was the choice when the best graphics software was only avaliable for apple. Now that most is avaliable for PC there isnt much of a reason to get a mac. PCs run all the software better, lol.
Save yourself some bucks and get a more powerful computer, get a pc notebook that has a Pentium M processor and at least 1gb of ram. HP and Dell are the best choices at the moment. If you have some serious bucks to spend get an IBM Thinkpad T43. IBM makes by far the best laptops for work/buisness use.
I am a Graphics Art Graduate but am currently working in an IT department. My company uses Apples for all their graphic design needs and are very happy with them. If your grandaughter likes using an Apple, buy her an Apple. If she likes PCs, I recommend buying a Dell Latitude D810 with the upgraded WXGA screen. This has a larger screen area and would be better for Graphic Design. Its what I use for my side business.
I fix Dells and I fix IBMs all day long. Dell has a better product and a better warranty right now. (subject to change)