Wow isnt very graphic intensive. upgrading your grpahics card is the last thing yeh should do for wow.

Might want to reduce some of yer graphics settings
Kill the shadows affects reduce the view range.

Did yeh set maxfps and maxfpsbk?
Did yeh set affinity?

Might also want to check and see how much memory yeh have left when yer running all yer accounts, vista require a considerable amount of memory just to run by itself.