Here's the thing. Your time is worth money. Due to their error all of the time you spent setting up your rig would be lost plus your time back and forth to the store, plus the general hassle, plus the fact you would have a "return mark" against you on an expensive item which could result in a 90- day return ban.
Bottom line, you would experience a financial loss if you return the item which you received due to their error. Is it ethical that you should lose time and money for their mistake? In this light, I believe there is nothing unethical about you keeping the unit they gave you. It's not like you went to buy a Jetta and they accidentally gave you a Porsche Cayman.
As a matter of fact, returning the i7 unit would probably cost BestBuy more in overhead than the incremental cost to them of giving you the wrong unit. Just keep it and make a point of doing something nice for someone this week.