Yea, bestbuy is limited. circuit city used to have a big selection of different recievers, but they are gone now.
A 5.1 and 7.1 reciever would work, it would just have wasted channels if your only using two of them. The impedance difference shouldnt be a problem. Just look at the specs. If it has a sub amp built in, look for its minimum impedence. if it just has a pre out for a sub amp the sub impedence will only matter for the external sub amp