Our sparky knew nothing re this stuff but gave it a go and stuffed up a couple of things. eg, he ran speaker wire for the subwoofers.
I assume you will have an electrician at some stage - apparently guild & spence are good at the home data/video / audio and they can also do your electricals. I wish I had have used them now although we have largely solved all of our electrician generated probs.
My main advice, would be to have a detailed plan up front. Specify every outlet, and every cable up front. It's a pain in the arse but well worthwhile.
Make sure whoeever wires knows about minimising interference between signal cabling and electrical wiring (there are some rules about running wires parallel to each other).
the benefits of cat-6 are great. eg, you can hook an xbox as a media extender in another room from your pc. The xbox will attach to your tv and stream music/video. tech is still basic (eg, can't easily stream dvd from harddrive via mce) but the next couple of years should show promise.
If you want IR distribution, be wary of mysky/sky boxes. some ir dist systems don't work well with mysky, and i heard normal sky too. google blue eye IR NZ for some info.
good luck. we are very happy with our system ,people are generally blown away - houses should be made at the least to feed cat5 or 6 through their houses.