To watch 1080p movies, would it be best to use an HDMI cable (I'd probably need to use a 25' cable) or would it be best to get something like Chromecast? I also have a 360, Wii U, and PS3 in the same room. I'm guessing that my wireless N router wouldn't be good enough for streaming HD content, but I should also mention that I have a TV with Ethernet and USB ports (though I'm not sure what formats it reads).
If Chromecast is the best method, does that use the wireless router to stream from my PC to my TV?
If you can setup your router to do that, I suggest you try that before buying any other devices. I stream HD movies to my tv over a Wireless G network with no trouble. As far as speed goes, Wifi N is more than enough as a protocol. What makes or brakes the streaming experience is the quality of the signal. You need a very steady stream of data to watch a movie without problems. You might have obstacles that degrade your speed (walls, doors etc.) or maybe the specific wireless devices you use might not work well together. I have had wireless adapters over the years that simply wouldn't reach good speeds with certain routers.