First off, Macs are not great gaming machines, especially not for the money. I have a relatively modest $600 PC that can spank any iMac less than three times it’s price. Not to mention that most games would require a Mac to go through Boot Camp, and that generally requires shelling out even more money for a full version of Windows. So to get what I have, you would need to spend at least a couple of grand.
And a Macbook (or pretty much any laptop) is a poor choice for gaming anyways since laptops have inferior graphics capabilities. Even my little old GT 240 video card beats the hell out of just about any mobile video chip (like the ones used in the older iMacs, or the Mac Mini). So unless your gaming is quite casual, you will be far better off with a desktop system, preferably one with at least 1GB of video RAM, which is more than any iMac except for the top model or just about any PC laptop.
PC gaming offers things that you cannot get on a console (and vice versa) so any serious gamer has both a PC and at least one console. For instance, I like the MechWarrior series, but the only one that was ever on a console was MW2 for the PS1 and it sucked compared to the PC version; it was virtually unplayable. And MW4:Mercenaries is PC-only.
@HungryGuy Not entirely true. My previous PC was a top-end gaming rig that my buddy bought in 2004 and it ran even the newest 2009 titles fairly well despite being technically obsolete. Granted, a 3.4GHz Pentium 4 and a Radeon 9800XT are quaint by today’s standards, but they did get the job done. That still-running rig got gifted to a friend when I upgraded and he manages to do okay despite hardware that is now older than my tray-loading PS2.
@Seaofclouds Definitely true, and more true for gamers than anybody else. Some people may prefer them for other things, but I have yet to hear any gamer that went on about how awesome their Mac was at gaming.
@johnpowell Viva la Hackintosh! For those that prefer OS X, I maintain that a Hackintosh is far better than an actual Mac.