i think the airport allows you to use a wired network, in addition it acts as a wireless access point.
to begin with you need storage accessible over a network, could be a NAS, a shared drive on an HTPC or a dedicated media server / streamer
then obviously, you need the network, if you plan to stream HD video, gigabit network is desirable (though not strictly necessary) - (Cat5e and 20m cable length from router makes me fall back to 100mbps on my main HTPC, but there has never been a stutter even with HD content). ensure cat6 for any reasonable length of LANcable. WIFI is nice, if you plan to put speakers / computers in a room, where wiring is a challenge, also wifi can help deliver a wireless multi-room remote, through a pocket pc / ipod touch. however to stream video, at least wireless N is required.
then at the recieving end you could use a DLNA compliant reciever or TV, or a media box like the popcornhour etc. the reason I suggested the airport was because it allows you to use any (exisiting amp+speakers). you could also use apple tv, but IMHO, it's overpriced and underspecced. obviously there are other brand solutions
at the upper end of the market, you could get an home automation company to do everything for you. like a poster described a few posts back. should have a lot of "wow" value with your friends.. we have a crestron system in one of our conference rooms, and it's pretty impressive.
if you have a handle on how much you want to spend, there are plenty of people on the forum who will help you spend it.
cheers
Edit: missed teky's reply. i just repeated whatever he said, possibly less lucidly