How do I transmit video from my iMac wirelessly?
How about an Apple TV ($99) and Airflick (free), over WiFi?  It'll only be 720p, though.

http://ericasadun.com/ftp/AirPlay/

There are a number of solutions for transmitting high-definition video:
https://www.google.com/search?q=1080p+wireless

...but to be honest, they cost nearly as much (or more) than an AppleTV, which gets you way more functionality.  Unless you have an enormous TV and top-notch content, you're going to be very hard-pressed to tell the difference.