Once you have win7 64bit, the memory upgrade will have the biggest affect on your FPS in dalaran.

One thing you don't mention though is your network connection spped. Having boxed on a laptop from numerous nets, I can confidently say most people underestimate the effect that net bandwidth has. In a place like dal, the server is sending quite a bit of data your way because of all the other toons NPCs etc around. Multiple that by 5, and it becomes a lot of data. Extra memory will help some by increasing the physical to virtual memory ratio each client is experiencing so that you VM system doesn't thrash but it can't fix everything.