Don't get a Mac if you plan on doing any gaming.
1) For the same price you can get a much more powerful Windows PC (componentwise)
2) Games for OSX won't run as well as their Windows counterparts, as less time is put into development.
Bottom line - if sheer power is what you need, don't get a Mac. I got mine because I love the fluidity of OSX, and the overall aluminium design - combined with the portability of my MBA. Not for the power though - for that a Windows anyday
Well why do you use a mac then?
For development? --> use a pc, you have visual studio and there are far more pc users then mac users
For video and photo editing? --> use a pc as you can upgrade your hardware and almost all the important apps also exist on windows
For music making? --> use a pc, you will have fruity loops.
Bottom line is, such an answer is a bad one, for all kind of usage I can think of a reason why a pc with linux or windows would be better, so stop telling people "don't get a mac blablablabla" Everyone has is own reason to get a mac.
I am also a gamer, and bought the 27" iMac for gaming purposes, I hear you say now why? Cuz I love the design and I prefer osx above windows. And if we would be honest, how many mac users also use bootcamp? more then 90% I am sure of it!
And if a mac wasn't made for gaming to, why would they lobby to make the games also for osx? Why do they sell games in the app store? Why would steam make a devision for mac games? Why did apple make full support for bootcamp?
So stop telling people what they should get, and let them have their own reasons for having a mac!
BTW I also agree that apple should make it possible to change the graphic card for the gamers.