Ok, I've read all of the posts about FPS and I've tried all of them.
I have even put the game on a Intel X25-e drive and sym linked it. It loads really fast and the disk drive is definately not the problem.
The problem was there before the drive and still there after the drive.
I don't believe I've ever tanked inside of an instance.
I tank when I leave an instance or go to Dalaran.
I don't think that two or three copies of wow should tank even if Dalaran is fully loaded with people.

Vista x64
I have a DQ45CB motherboard that claims to have a PCI-E 2.0 slot.
I have 8G of ram ( nothing fancy or overclocked 800M or something like that )
I have a Q9550 ( or a 3GHz quad core )
I currently have a Nvida 280 , although I have used ATI 4870 and then I have the old Nvida 8800.

of all of the combo's the Nvida 8800 never tanks my FPS while both of the PCI-E 2.0 video cards tank. I was down to 1 FPS and couldn't even hardly log off.
I'm only running 3 copies of wow on the tanking machine and at one time I was running two copies of wow and it still tanked.
The other machine I have never tanks either and it's currenty running two copies of wow and an ATI 1800XT.

I supposed the only other thing that I can think of to compare is that the tanking machine has two monitors connected to one video card.
I only ever used one monitor on the tanking machine when I was using the NVidia 8800 ( the NVidia 8800 was able to run 4 copies of wow with the FPS limited to 15 )

I suppose I can try using one monitor and see if it still tanks.