The scope of this question is a little bit beyond this forum (AVSforum may be better ) as there are just so many variables but in general:
There are two components, the wireless network itself and getting the tv signal onto the network.
For a wireless video network you really need to think about a dedicated setup that isn't shared by every other device in the house. 5GHz-N networks often work best as they have the least interference from other devices however you can make a normal wireless N work if you pay attention to distance, use directional antennas etc. Distance is the primary factor that will determine how many HD data streams you can support; one is easy, multiple requires more attention to detail.
The second issue is getting the video signal onto the network. Depending on what you want to use as a source device, a sling box works fairly well. If you are dealing with unencrypted ClearQAM content, an HD Homerun is excellent. But be aware the industry is moving away from non "copy-protected" and non-encrypted content making media distribution far more difficult that it was in the past.
The combination of Windows 7 media center and Xboxes as media center extenders can handle just about any content (even most encrypted) using cable card tuners. MythTV and a cablecard tuner can handle content that does not have too many copy protection flag restrictions on it.