The multithreaded OpenGL optimizations for WoW have not been patched in. Thus, you aren't seeing any magical performance improvements, unless Apple upgraded the graphics drivers in 10.4.8.
As for your performance increase, again, that most likely has to do with you looking at different parts of the game and having the options screen drawn/not drawn, etc. Also why don't you have shader effects enabled?
Also, you are most certainly GPU limited, not CPU limited. The Core2Duo can easily push a 7950GTX to its limits, let alone a 7600GT. Don't get me wrong, the 7600GT is an awesome card, and finally a card worth having in the iMac. But overall in terms of game performance, its still holding your CPU back.