How important is it and what does it mean? I am running a standard 5.1 system and do not plan on going the 7.1 route. I only use the receiver for movies. Do I really need it to be THX certified?
No it is just a name. Companies paid large sums of money to George Lucas to become certified. There are standards, but basically they can buy their way into it. In fact, many receivers high end enough capable of being THX certified aren't because the manufacturers don't pay for it.
I heard that George Lucas sold THX, so it may even lose its credibility and 'prestige'.