On the 500 you'll have to get an additional amp to power the extra two speakers, also I think you'll also have to purchase an active sub-woofer to replace the passive one, plus obviously the cost of one or two extra speakers.
It's one of those things, if you've got a suitable amp knocking around and were planning to get an active sub anyway then why not go for the additional speakers. However, as very few films have "true" 6.1/7.1 mixes then it would probably be an expensive route for probably not huge audio returns.
I say this from the perspective of someone who hasn't got 6.1 or 7.1 so others may disagree.