While i disagree with Sanjay about the 7.1 vs 5.1, his points are not without merit. I'd check to see if you can return speakers easily, and if so, test both the 5.1 and 7.1 setup in your room, and go with the one you like best. Even if you have to pay some shipping costs, it'd be worth it to ensure you end up with the best solution for your room.
I do agree with Sanjay about the choice of speakers - Better to have speakers that are as closely matched as possible, especially for the front L/C/R. It made a big difference in my situation, and i'd not want to go back from 3 matched fronts.
"He who asks feels dumb for a few minutes, but he who does not ask remains dumb forever."