I play WoW on two computers, a Mac and a PC.
The PC is:
P4 1.5Ghz
768 Megs RAM
Radeon 9600 XT, 128 Meg
The Mac is an iBook. It's supposed to be my work machine, and it is configured as follows:
1.0 Ghz G4 (newest generation of G4's)
768 Megs RAM
ATI Radeon 9200 Mobile, 32 Meg
I primarily play on my PC, but I've played a few times on my Mac. I love my Mac - and it CAN play WoW, but I get 9-12 Frames/sec.
At work, my father uses a brand new iMac G5, and it plays WoW at a solid 45-50 Frames/sec. Of course, for the $2500 that computer cost, you could get a gaming PC that could outperform it.
Macs are excellent systems. They are technically and functionally far more advanced than Windows boxes. However, while they are capable gamers, they are certainly not the best value in a gaming computer. If gaming is your goal, buy a PC. Or, be like me and get both :)