Buy a TV with 1080p and you will be covered for some years to come. Most channels aren’t even being broadcast at that rez yet, Fox is 720p. I think the others are 1080i.
The cost - well that can go as high as you let it.
Receiver - You must have a DTV HD receiver. Do you want a base HD receiver or do you want a built in HD DVR. The latest DTV HD DVR is a pretty nice unit. Way better than their last one. The latter is way more expensive and kind of hard to get now.
DVD Player - Minimum that you need is a progressive scan DVD player which is cheap and you call go all of the way to Blue Ray. This is the area where I am being more cautious with which way I go. Its a good excuse to get a PS3 and they come with a BR player and you get HD gaming to boot.
Local HD channels are free with an antenna.
DTV HD channels are a package - I think its like 6-9 bucks a month for their base channels and the premium ones (HBO) adds more.
If you have the Sunday Ticket (NFL) you have to buy their Superfan package in addition to their normal superfan package, if you want HD football.
Heres a pretty decent website to help learn more - good for all electronics for that matter
http://www.cnet.com/4520-7874_1-5102926-1.html?tag=prmo1