There are loads of options but first some fundamentals ought to be covered;
a) As you are transmitting data _Locally_ within your home network, the speed to the internet provided by your ISP (TalkTalk) is not relevant.
Its the speed which your computers (Desktop, laptop & hopefully a media centre PC) can transmit data between each other on your _Local_ Area Network (LAN) that counts.
b) HD Video needs significantly more bandwidth then Audio streaming.
2 of my 3 desktops have "108" Mbps max (c 40 Mbps real) wireless capability using MIMO & pre-N protocols which provides more then the standard "54" Mbps. They can stream Standard Definition (SD) video, just about smoothly.
But my laptop with standard built in "54" Mbps suffers from occasional drop out and "stuttering" with SD video.
I only mention this due to your HD TV objective.
For HD, realistically, you will need cabled connections and ideally at 1Gbps rather then the current 100 Mbps capability most commonly available via routers, Ethernet ports, etc.
c) Audio streaming has proved to be fine, even with my laptop's standard "54" Mbps wireless, but that is helped by the capability of my MIMO wireless router.
I have an HD TV and am also aiming to get a media centre PC for it. At the moment, I make do by connecting my laptop to it.
It feels really unreal doing emails on a 40" screen! Seriously though, photos and videos comes out a real threat at 40".
d) Wireless security is essential in my view. But that is another topic in its own right. Do a search of threads in this forum and there will be loads of very useful information you can read.
In summary, yes, wireless audio streaming is easily achievable. Even setting up wireless security should not throttle your wireless LAN bandwidth enough to matter.
Kit is fairly standard and reasonably priced; wireless ADSL router c£80, <£10 for a Ethernet PCI card for your Desktop (if it needs one) to connect to the router, your laptop presumably already has WiFi, as would your new media centre pc.
Video streaming requires cabled or wired networking, realistically. Especially as you ought to look to the future. However, newer wireless technology might make it to the consumer mass market as soon as 2 yrs from now.
But Ethernet cable (cat6 or cat5e) for a whole network in a normal UK sized house would struggle to cost more then £25 via web vendors.
Going cabled means you don't have to pay at least twice as much for a "turbo-ed" wireless router and £60-70 for EACH "turbo-ed" wireless adapter for your laptop and media centre pc.
Hope that helps and is a good start to your quest.