I think you'd have to run BootCamp or something to use ISBoxer on the iMac you're getting, but don't quote me on that. I'm sure one of the ISBoxer gurus will show up and give you more info on that if they aren't all gorging themselves on Christmas dinner yet, lol.

As far as if your current system can handle it, you can go to the WoW site and sign up for 5 trial accounts and use those to test just by running as many accounts as you like. Keep in mind, however, that multiboxers do change a lot of settings that will net you better performance, such as running the slaves on low settings and using the FPS sliders in game to cap the slaves to a low FPS. My slaves use 13 FPS for background and 25 FPS foreground, but anywhere in between will do. My master is capped at 70 FPS.