If you're 15 and interested in running professional software programs, congratulations!! ? You're motivated AND smart, so don't let anyone tell you otherwise. If you are thinking of graphic design or web design as a career, you can acquire you software for a few hundred dollars instead of thousands because you get a student discount with student software sites. I wouldn't waste my time with any of that iStuff if you are into this for career.
Down the line, if you want to do web design, you will want to work on windows OS because the majority of the world does and you want to cater to the largest audience of prospective customers. I've been a Mac user for over a decade because when I was in college someone said "the graphic design industry chooses Mac overall." Well, Macs are good, without a doubt. However, I'm making the change to Windows because I just can't justify getting a Mac. Yes, you can get Parallels and run Windows Vista in a window, or you can partition your hard drive and run one OS at a time in bootcamp. I don't see, however, why you would NEED to spend extra when you could just get a quality laptop with Windows Vista and have all you need.
The only offices I have worked where I have been offered a Mac for web design is in a design firm geared for print with a manager geared for print who knows a web site is necessary for the client though does not know too much about HTML or CSS. I found it to be a waste, and it costs time when you have to return to something to make it look better in Windows. Chances are it will look great on a Mac and might not look so good in Windows. However, if it looks good in Windows, the likelihood of it being good for the Mac also is quite higher. The two top browser priorities are IE7 and Firefox, most people ignore or place little emphasis on fixing Safari issues.
If you want to do print only, I say either Mac or Windows is fine. Not all print people use Macs anymore because computers have grown to be better overall than they were when the design audience latched onto Mac. ? Most important, you want to have enough memory. I'm getting 2GB. You should not get a machine that will not allow you to expand to 2GB in the case you only purchase 1GB to start, though 2GB is better for multi-tasking.
As well, if you are interested in design with Adobe programs, I recommend spending some time on the Adobe forums as well. You can get answers to your how-to questions and might find more insight form Adobe users. By the way, Fireworks CS3 is something I can not live without. I spend more time in it than Photoshop overall ans I get tons accomplished faster. There are free webinars that you can attend to help you get an edge on the new features as well.
I'm in the process of picking my laptop, so I can't give you the one ultimate answer for THE laptop for you. If you get a 17 inch, it is a bit more to carry on a daily basis. I've been using a 15 inch and I'm switching to the 17 inch because I want to save space on and around my desk. I do not plan on carrying this with me every day of life, though it will travel with me about half the time.