OK, so to respond to the other questions...
For subwoofers (and speakers in general as a matter of fact), the specs are only useful for a very basic comparison. They are not anywhere near enough to determine which is the better one, especially as the method of rating a subwoofer's specs varies from manufacturer to manufacturer. The best way to do it is reviews and listening - I recommend SVS and Velodyne because I have heard both, and they are both excellent. SVS gives a better value, hence my recommendation for their PB13 ultra.
You do need a receiver for the system pretty much regardless - in the case of hooking it up to a computer, the cables would go from the computer to the receiver to the TV and speakers. All modern receivers can take HDMI as well, so if your computer supports HDMI out with sound, you could just plug one HDMI cable from your computer to the receiver, one from the receiver to the TV, and the receiver would do the job of stripping the audio off of the signal and sending it to the speakers (along with all of the amplification).
Next, for the question about the 802Ds - what do you mean by 3? They aren't really designed as surrounds, nor are they designed to be a center speaker, so even if you decide to go for this, I would just get one pair of them (for the front left and front right). Bowers and Wilkins makes a center speaker designed to pair with these called the HTM1D. This is sort of like an 802D on its side, so it easily fits below the TV without blocking the image. For surrounds, there are other speakers designed for that purpose, such as the DS8S. However, as I said, I am partial to the high end Bowers and Wilkins. Other people like the sound of other speakers, and that is why it is so important to hear them before buying if possible. Also, don't shy away from listening to lower end speakers as well. You might be perfectly happy with something like the 703 or 704 that I mentioned above, and if you are, there's no reason to spend 5 times as much for the 800 series speakers. Also, take your own CD to listen to - that way, you know what it should sound like, and what you like. Listen for a decent amount of time, and listen to as many speakers as you can. This really is the best way to guarantee happiness with your final decision.
Oh, and receivers are perfectly happy driving fewer channels than they have. I have a Denon AVR 2808CI (7 channels, 115 watts per channel) driving a 2.1 setup right now (a pair of Bowers and Wilkins 685 mains, with an SVS 16-46 PC-plus subwoofer). At some point, I'm hoping to upgrade to full surround, but for now, this setup is working great. As I said, the brands I'm recommending are ones that I'm using right now, and I love them. Specs alone are not sufficient (although for receivers and amplifiers, specs do a much better job of describing the quality than they do on speakers - there is less variation in sound quality between amps than there is between speakers).
Oh, and if you do get to a decent high end store, don't let them talk you into spending too much on cables. A decent speaker cable should not be a tremendously expensive device, and if they are trying to sell a $500 cable to you, it is not worth the money.