back up your WoW WTF folder, inside the folder is a file called "config.wtf". Delete this file to reset all video settings to default and set everything on low (for testing purposes) for each client, exit WoW and restart the clients again.

ground clutter, water effects, sunshafts(sunbeams?), shadow quality, view distance, texture filtering and environment effects can cause major fps loss if set too high.

if you upgraded to windows 7 over vista instead of formatting hard drive and installing windows 7, make sure there are no left over programs/services from the vista install running. press ctrl+alt+esc and see what is causing all the memory/cpu to be used. keep in mind if you have prefetch on, the memory will be listed as low and most of it cached, this is ok. turn off or alter the "windows indexing service" so that it does not run constantly. this indexing service can cause high HDD usage and thrashing, also make sure you are running in performance mode for "power settings"