I run 12GB of RAM on a Core i7 system, and while my memory usage when 5-boxing is usually in the 4.5-5.5GB range, in some places (Dalaran is an obvious example) I've seen it spike to 7.5GB. This could be why you're seeing such lag in zones with lots of people (and thus, lots of textures to load). So I would also put in a vote for doubling your memory to 12GB.