Isn't this specific to the 360? I don't remember a switch on the original xbox hd adapter. I could be wrong though...
Doh! I thought he was referring to the 360! For the Xbox 1, things will be a bit more difficult. What size is your TV, ATB? What technology does it use (CRT, DLP, LCD)? On smaller, CRT based HDTVs the difference can actually be pretty negligible between 480i/480p. For larger sets though, it should generally be very obvious.
There aren't many games that support "true" HD (720p/1080i) on Xbox 1. KOTOR 2 does not- it supports 480p only.
For a listing of what games support what resolutions, visit:http://www.hdtvarcade.com/
As for your darkness issue- I cannot stress enought that you need to get your hands on either Digital Video Essentials or Avia. These are DVDs that are specifically designed to walk you through calibrating your video settings. After the chunk of change you had to lay down to get an HDTV, it would be a crime not to spring the additional $25 to get one of these DVDs.