If you're using Keyclone, you can use it to assign processor affinity. To do it manually, open Task Manager (right-click the taskbar and select it from the menu). Select the Processes tab. Highlight the application you want (WoW.exe) and right-click on it. One of the options is "Set Affinity..." and selecting it will provide you with a box that lets you select an available CPU via checkboxes.

As for memory, separate WoW instances take up around 240MB on my quad core system with 2GB memory, or just under 1GB total. If you are running 4GB already, I doubt that you'll see any performance boost from increasing it to 6GB.