Apple has explicitly stated a lot of things over the years - especially things like maximum RAM supported by systems, minimum hardware required for software etc.
The only issues you might have would be the same anyone using a 4870 on a newer machine has. For what its worth, you convinced me to start up WoW yesterday. Everything was smooth as silk. I didn't really do much though, just wandered around Dalaran a bit and then sat in the Stormwind Cathedral and spent about two hours sorting out my talent points and getting that new dual spec thing going.
Oh man. Sorry about that. (And at the same time, I hope you enjoy playing) I am planning to use this summer to get my wow fix and then really lay the game to rest. As I'm sure you know, it is way too addicting to be healthy.
That said, I love it.
Just out of curiosity, what server do you play on. I know there is about a 1/250 chance we play on the same one, but it's worth a shot.
Back on topic, given the stats for the machine I own, you think the graphics card is probably the bottleneck?
Here's a slightly more detailed rundown.
Processor: 2 x 2.66 GHz Dual-Core Intel Xeon
Memory: 4 GB 667 MHz DDR2 (4x 512 Sticks and 2 1GB sticks)
Graphics Card: ATI Radeon X1900 XT Mac
I assume those are the big three factors that affect gameplay. Aside from one's internet connection.
With that hardware, the game runs fine. However, I often notice framerate hiccups or slight dips especially when flying, in cities, or fighting lots of enemies. I have it locked at 30 fps and it stays there 85% of the time. However, I'd love to be able to see it at least stay at 30 fps 100% of the time if not higher.
I'm not in Northrend yet either. I did noticed, when I had a mage port me to Dalaran for some skill training, that the framerate stayed in the 25 fps range. That's what sparked this whole idea of getting a new card.
As stated, I run the game at 1680x1050 with everything on max.