many people face the same problem like yours and they can't find the root of the problem...
I've looked into many forums (including wow and toms) in the past looking at what causing lags of WoW. In theroy, any modern computer should obliterate WoW but even people with setup like the core i7 975 and GTX 295 in sli still lag just as much as your seeing.
I'll quote my self from another thread that was asking the same thing.
In my personal option, i think it's WoW network, severs, or just bad programing as there are plenty of people with this problem that have far more powerful systems.
As im reading in WoW forums like this thread, http://forums.worldofwarcraft.com/thread.html?topicId=16473762099&sid=1&pageNo=2
Something tells me there some bad programing going on.