I am a PC guy myself, but dont let me bias your decision.
When it comes to World of Warcraft the one technical feature that has impressed me is that they engineer it to be as compatible with older hardware as possible. They dont make an expansion that means you need to upgrade from a dual to a quadcore for example as well as have to replace your 2 year old video card. While you may not be able to max out your settings over time, with this game you should be able to slowly select lesser and lesser graphics requirements over say the next 4 years or so and probably on the 5th year be looking towards a newer graphics adapter. My one friend has an older iMac with GeForce 9800 and he is able to run cataclysm on max settings with a frame rate between 20 and 60fps depending on what is going on in the environment that the graphics card is trying to render. However I have a spare laptop that is a Core 2 Duo and GeForce 7300 from 2006 that can only play the game on lowest graphics settings otherwise the frame rate is a crawling 4 to 6 frames per second which is unbearable. On the lowest settings with the GeForce 7300 on that older laptop I get 15 to 20 frames per second which is ok, but not the best.
The iMac is the way to go if thats what you really want. My preference is PC because I play lots of games and dont have to worry about compatability issues with the games I play which are designed for PC vs ported for Mac OS execution. I also am not an average computer user, I will mix and match parts to get better performance and save from lining Apple's wallet with their markup. I like PC's still being very universal in construction which makes them easy and cheap to fix as well as upgrade. If your talking security, other than Linux, Apple is the way to go in my opinion.
