Advice given to me by a professional was to put in the biggest one you can. :)

When we reconstructed our house, I took his advice put in a the large Channel Vision 38" panel in the utility room, and ran cat-5e and RG-6 all over the place. Basically each room has two outlets - each with two cat-5e cables, and many also have RG-6. In three locations (living room, family room, and master BR) I also ran structured cable (2 RG-6 and 2 cat-5e in one housing) as satellite prewire. This centralized all the phone, net, and video stuff. The 38" panel is easily big enough to hold and hide everything, but I wouldn't want to add too much more functionality. Case in point, our security system is co-located in a separate, small, enclosure.

I probably have about 1/3rd of a mile of rg-6 and cat-5e in total. This is not overkill. When a friend of mine did his house, he took the opinion that each wall should have at least one outlet. He guesses he has about 2 miles of low voltage wire behind the walls.

Make sure you have a dedicated 20 amp circuit to a couple of key locations where you might have power amps for stereo/home theater. One for your computer server too.

For various reasons, I didn't do any whole house audio or fancy lighting. That would have added a lot. At that point, some start looking at installing relay racks for the additional gear...

I do think that installing some speakers in the walls/ceilings of some rooms with local outlets is a good idea. I didn't do that either - but I have a crawl space under the house. So it won't be too hard to add after the fact.

This link is to the outfit where I bought my panel and wire. They have a nice FAQ on this very topic.

Here is a link that might be useful: I am remodeling or building a house, what wire should I put in?