I run WoW on Mac OS X, with a Core Duo iMac with 2 gigs of RAM and I get good performance, overall: 30-40 FPS almost all the time. While raiding I sometimes drop to 25. I keep most of my settings down. The only place that's really bad is Dalaran, where I can dip to 15 during peak times. Play with your graphics settings for best performance.

I recently tested WoW on my brother's 9400m Mac Mini and was pleased with the performance- though his unwillingness to give me much time led me to only run around Ironforge for a bit though. He also only has 1 gig of RAM. Stacked with 4 gigs (which I believe is the Mini's top RAM now), i could see it being very nice.

Personally, if I were you, I would hold off buying a desktop Mac for a short while. I think the Core i7 will be debuting end of summer. But the lappies were just refreshed at WWDC, and I think they're fantastic. I got my hands on one to inspect and they're rock solid machines, good graphics, Firewire, and the battery is incredible. I couldn't run WoW on it but if it runs as good as every other Mac with the similar graphics, you should be fine.

But general rules apply: As much RAM as you can afford, buy with stock RAM and buy it after market. It's easy to install in a Laptop and you'll pay a lot less then Apple's preinstalled. You also will get the system faster.