7.1 versus 5.1 is simply adding two additional speakers (it can also be one speaker, centered on the back wall, for 6.1) to add some extra rear surround effects to fill in existing gaps. The 7.1 in HTIB systems is now popular because of the "numbers race," more marketing than real improvement. Many people inexperienced in home theater think it is a big difference over 5.1, not necessarily so. If a manufacturer made a 5.1 and 7.1 HTIB system, identical except for the two speakers, two amp channels and price, I would go for the 5.1 system for you. If you get the Onkyo system, the two speakers and 6th and 7th channel on the amp will just go idle, no problem.
7.1 is most effective when you have a wide area in the rear of the room that is not completely covered by the two regular surround speakers. Your system should sound fine in 5.1. Make sure you get the 2 surround speakers up at least 5-7 feet off the floor. Keep them out of the corners and at least a foot below the ceiling.
Good placement is slightly behind the listening position facing in towards the middle of the room (or slightly farther back towed in towards the front) or on the rear walls facing forward and in several feet from the corners.
Hope this helps.
I'm looking to purchase an HTIB and I like the ONKYO 780 7.1 but I only want (and have room for) the 5.1 speaker setup. The question is, If I bought the 7.1 HTIB and only set up the 5.1 speaker setup, would I have problems, such as loss of sound that is trying to go to the two speakers that I wouldn't hook up?