(And yes, I am now in the habit of shutting down Opera before starting World Of Warcraft on my Mac, I get better frame rate that way).
I actually solved that problem by going from 2 to 4 gb ram on my Vista64 machine. Before that e.g. landing in Dalaran made the game basically unplayable for a minute or so, and alttabbing to a Firefox window that was open in the background with 20 Wowhead tabs opened was taking somewhere in the 2 to 5 minutes range - seriously. By adding the additional 2 gb of ram (actually I swapped the ram, to a faster CL setting as well, but that's a minor detail, I guess) I was able to boost my framerate from typically 1-15 fps in Northrend to 10-40 fps, and be able to alttab to Firefox in 1 to 2 seconds, and landing in Dalaran now doesn't cause hiccups at all.
So you might actually just be at the limit of your hardware as well?
To get back to the original topic, though: I felt a much bigger performance hit when going from TBC to WotLK then when going from XP32 to Vista64, all on the same hardware.
On the SPEED channel the Truck Universe show is now playing an episode on Alternative Fuels: How to make your own biodiesel with the Fuel Meiser system.
"No matter where you go, there you are..." -- Buckaroo Banzai