Are you running with max video settings?

Check keyclone setting to see if you have reduced the FPS of non active windows.

This might seem a daft idea but try removing one of your gfx cards and see if you get the same issue.

I've heard of a few problems with SLI cards.

I run an old 8800 GTS , quad core and 4 gig of ram and i can run 5 characters just fine.

As for your question regarding a 2nd hard drive, i dont think your problems have anything to do with space / drive performance.

Also if you have 4 gig of ram at the moment, adding another 4 gig of ram wont help at all. Each copy of wow will only use 300-400mb of Ram anyway so you still have loads left over. It would be wasted cash.