I think your hardware choices, from a HTPC standpoint, are overkill. So you are completely fine there.
I particularly love the 7600's as they have all the latest and greatest postprocessing features from Nvidia. Just be sure to match it with
Nvidia's DVD decoder (you only need the cheap version if you end up using spdif output from your soundcard). HDMI on a video card is nice. You don't HAVE to have it as DVI works the same but gives you more options depending on what inputs your TV has.
As far as a sound card be sure to check
this Chaintech AV-710 7.1 out. It's very higly regarded everywhere I go. Unfortuantely I don't think newegg sells it anymore so you'd have to find another vendor. I consider myself a hardcore gamer. I've always had pretty good peices in my game box but I've found spending big money on sound cards to be the biggest waste of money. Currently all my sound cards are onboard audio. The Abit motherboard I have in my HTPC has a spdif output so I don't have to worry about digital to analog converts and the like. The soundcard basically is just passing the audio thru to my receiver. I think you only need to worry about the quality or $$$ of a card when you want to use its analog outputs, or perhaps some other hardware feature that again, I've found to be a waste.
Briefly on the AVR, I'd say you are right. With a HTPC you will be doing most everything in one box so video switching is pointless feature on the AVR. In your case you may have to keep the cable companies box, but you could just have it and the HTPC connect the video directly to the TV and just send the audio to the AVR. Get a Harmoy remote (if you don't already have something similar) and call it good.
Here is a link for a bundle to get OS, DVD decoder, remote and SDTV tuner together as an example.
Otherwise I'd say order those parts and get that HTPC running!
