All of the advice above is good, so please excuse me if I question one aspect of your thinking (and I don't mean this as a personal insult in any way). You didn't mention any near-term plan to get a larger listening room, so I don't see the merit of a 7.1 HT system in a room that measures 12'x13'. Infact, I think the 7.1 HT setup is questionable for the vast majority of home listeners for two reasons:
1. there is very little software being made for either 6.1 or 7.1 format, and from the articles that I've read there is little intent in Hollywood to do so;
2. adding several more sets of speakers, plus the required amplification, adds a lot of cost and provides little or no improvement over a high quality 5.1 system (unless, perhaps, the listening room is quite large).
With regard to point #1, I suggest you go the following two links on Bryston's Web site and read the comments about 5.1 vs. the expanded formats:
1. http://www.bryston.ca/newsletters/55_files/vol5is5.html
(see the article called "Encode vs. Decode")
2. http://www.bryston.ca/newsletters/64_files/vol6is4.html
(see the article called "The Great Capitulation")
My personal advice is to stick with the 5.1 HT setup and invest more money in the quality of your amps and speakers, probably adding a good subwoofer, before adopting the expanded formats. Sean's recommendation to get a decent 3-channel amp (such as the Adcom GFA-5503) for your center and surround speakers makes a lot of sense (and is the approach I used in assembling my own HT system). Investing in better quality 5.1 format equipment will result in a much better HT system than adding several more mediocre speakers and channels of amplification.