I just got my new HD system. I have a THX 7.1 ready receiver with a 5.1 speaker setup : Polk audio RM6900.

Is it a good move to go to 7.1 or I will see no difference compare to my actual 5.1 setup ?

If I decide to go 7.1, I just have to buy 2 more satellite speakers (polk audio RM6801 in my case), put the speaker behing the listening position and that's it ?