PSU, videocard, then get a SSD to host the WoW/Data directory (or the entire folder if you like). Q6600 is still a good CPU for multiple instances of WoW and you'll see a better benefit from faster texture fetching on a SSD. I ran 6 instances of WoW on an E6600, and adding the SSD back then was a good performance boost in busy areas and an easier/cheaper upgrade than moving to the i7-965EE/X58 platform.

Videocard will help with overall frame rates.
SSD will help prevent stuttering from fetching too many textures at once (like when many players are entering the area - BG/city areas or flying fast over terrain).
Slower quad core to faster quad core will slightly help run 4-6 instances of WoW, but I'd bet video/disk access is your real bottleneck.