To put some closure to this, I upgraded the cpu/mobo/memory yesterday. I went to a i7 2600k, 8gb 1333mhz memory on a new asus mobo - Will be upgrading to 16gb next check.

Same video cards, same hard drives.

Single client framerates fluctuate between 150-200 in Orgrimmar with 1 client running. With all 5 it caps out, 60fps on the primary window with video effects 2 steps up from the minimum, and 30fps with video effects 1 step up from minimum.

CPU utilization hovers around 20-30% at all times - it absolutely positively was the CPU. Thanks for all the input.