Why is it I can run 1 copy of wow or 5 copies of wow and get about the same damn performance?!

Anyone know of a way to unlock this potential... I just dont understand why my machine doesn't absolutely fly when I am single boxing and raiding...

Spec's:
Athlon X2 6000+
2GB DDR2 (Crucial)
nVidia 7900gt (Dual head in use) (256mb)
nVidia 8500gt (Dual head, not in use at this time) (256mb)
Onboard Sound

I've considered buying a soundblaster to offload the sound, but does it make that much of a difference?