WoW is a very CPU hungry application. On a single core chip (like my old 640) a single WoW client would utilize 90% of the processor. Running 4+ WoW clients on a dual core chip would definitely see a bottleneck at the CPU.

That said, I would also endorse the Q6600 (or more expensive QX's) as the upgrade of choice for nearly any system used for multi-instancing. Just remember to set core affinity either manually via ctrl+alt+del or using an automated tool like the one found in Keyclone.