I'm gonna second what Elga said with a few more details. First, for gaming don't bother with a laptop unless you actually need a laptop. Actually that goes in general, not only for gaming. Portability is fun but they cost more and are much slower for the money. Laptop components are, for the most part, slower and less power-consuming versions of what they put in desktops. This tends to be true for the motherboard, processor, ram and hard drive. While you can get a powerful laptop which isn't as slow as I mentioned, most of the time you'll have to pay a large premium and it won't come close to the speed of a desktop for the same money.
Where gaming is concerned I 1/2 agree with Elga. It really depends on the games you're going to be playing. Because you can run Windows on your Mac nowadays, the operating system isn't the issue. The issue is graphics upgradability. Do you need a BEAST system with SLI or Crossfire? If so, then you have to do a PC and I agree with Trish, building your own system can be fun and educational. The drawback is if/when something goes wrong you need to replace that component rather than take the system in for warrenty repair like you can do with Apple, Dell, etc (not promoting Dell, no offense to anyone but most brands, including Dell, strike me as overpriced junk).
So, are you going to be playing Fallout3, Oblivion, F.E.A.R. 2? (BTW, if you are into RPG's you MUST get Mass Effect). If so, having a computer where you can set everything to MAX while still maintaining a high FPS is rather fun. I don't know if you read the thread about Sadie's laptop but I still stand behind everything I said there. My PC gaming system is my secondary computer. I tried making it my primary.... and the lost productivity wasn't worth it. Obviously the stability issue is less essential for a 9th grader than the primary income-generator of a family (my computer dies and so does our income!). I don't mean that in an offensive way, just that you have some flexibility. I went through my share of crappy computers in high school and college and learned a LOT about how to fix software and hardware issues on PC's.
If you are willing to put up with a higher headache ratio and are interested in learning more about computers (and possibly spending way too much money on graphics cards!), I would go with a PC. It still provides the best gaming experience. Also, PC computers tend to be much more upgradable over time. Even with the Mac Pro you can't just pop in a new processor when the mood strikes you, though I have done that a number of times on my PC systems. And the iMac and mini... those computers actually laugh at you when you use the word upgrade.
If you want something easy that just works and don't need to set your games to MAX graphics settings then go with a Mac. Another benefit of a Mac, if you have any interest in having an internet-related career, Macs have a lot of standard Unix tools built in which are very useful to learn. Whether you want to be a programmer, server admin or graphic designer, being familiar with Unix is very helpful in the job market. With a PC you have to install Linux/BSD and the user experience will vary drastically per distribution and your skill level. With a Mac you get the Unix goodies with a stable and easy to use system, best of both worlds in my opinion.
Josh
I always had a hard time with that step.
