Your motherboard/chipset is the core of your system. Without the underlying FSB speeds and well-written quad-core support, you won't really take advantage of the Q6600. For the money, though, the X2 CPU upgrade will be the biggest one-shot in the bunch. You'll definitely get a much larger bump with the full upgrade but the CPU-only route is definitely a worthwhile option assuming your current motherboard supports it. For running 5 WoW clients, though, you will definitely want to plop another 2GB of RAM in that thing. Prices being what they are, you're probably better off either waiting until you can just upgrade it all or swapping in the motherboard, CPU and RAM first until you can afford the rest (assuming money is an object).