WoW's graphics aren't particularly challenging, and this is intentional so the game runs reasonably well on low spec systems. But understand that it *is* fairly demanding of processor and memory - especially in crowded areas.
Since you have a multi core processor, there is a way to tell the game there is more processor available to use. Essentially, you go into the config file (WTF folder: config.wtf) and change the affinity value to better correspond to the processor type you have. Open the config.wtf file with your favorite text editor. There is an entry near the top called:
SET processAffinityMask "*"
where the * is a number. The possibilities are 3 for a dual. 7 for a Triple, and for your Quad, the max number it can use is 15.
(for the i7 guys: 31 for 6, and 255 for all 8) This is different from telling your OS to pin the game to a given core. What it does is tell the game how many cores
(virtual or not) are available to handle whatever threads are available.
Be advised that simply maxxing out the value for your CPU isn't necessarily the most effective approach. So for your Quad, you might want to try 7 instead of 15.
Check these threads:
From the Warcraft Support Forums:
http://forums.worldofwarcraft.com/ [...] rhtml=true
AnandTech:
http://forums.anandtech.com/messag [...] id=2310148