I just upgraded my memory, and I do not agree that memory speed is far more important. I have DDR2-800 and no plans to increase that anytime soon. My CPU is definitely the bottleneck at this point since both cores are pegged at 100% when I'm anywhere with other people. This also explains my FPS limitation while single boxing as CPU clock speed affects max FPS (oddly enough). When I overclocked my CPU from 2.2GHz to 2.53GHz I gained over 15 FPS, so I would imagine going up to 3.1GHz from there would give me quite a significant boost, and hopefully ease up the lag in Dalaran as well.

Plus upgrading to a dual core 3.1GHz athlon is actually fairly cheap $73 - whereas any intel processor near that same clock speed is >$200.