In general you can just turn them all on.
You didn't tell use what TV you have or its native resolution which is the only way to determine if 720p or 1080i is better.
As for dark...haven't seen it. My HDTV looks great. Most of my games are only 480p though, as are the games you mentioned. Amped2 however, is my fav. 720p enabled game.
Remember that your HDTV has completely different brightness/contrast settings for not just each video input jack but also each HD mode. So if you carefully adjusted the brightness at 480i, it won't mean things aren't too dark at 480p.