Considering 27" iMac. Can I use it as a TV?
The best way to get TV hooked up to a Mac is through a TV Tuner. The best TV Tuners for Mac are by far the EyeTV tuners. The TV tuner could also be used to hook up to your Wii, however that is a bit impractical. Instead consider buying an adapter from RCA to Mini-Display Port to use your iMac as a Monitor for the Wii.
I don't believe that is an input, but only an output, for a second monitor.