I myself. do not overclock. Not needed in WoW but it is up to you and your skill level.
I don't know much about the new Nvidia GX cards, but that should be plenty to run 5 toons.
SSD will just cause things to load faster. Up to you to justify the added cost.
Looks like a good build, my first upgrade would be get 12 gigs RAM but 6 will 5 toons