Nice Site..... There are a few HALO themes out their you might want to take a look at. I like the off topic chat as well, it will help expand your network of sites in the future as people move from game to game... keep a close eye on it and see what will be the next hot game to create a forum for. Good Luck... I think you will have a good site going there
I like the header design! Very nice For the bars in the template area, why not use a more saturated/brighter orange?
nice colors and sleek web2.0 style, although I'm not sure they "connect" with Halo itself But then again, that can set you apart from other boards that use the typical colors like Blue/Green/Grey. Good idea having a dedicated subforum where you can post trailers and other videos for Halo 3. I would recommend adding some graphics to the header and really focus your efforts on Halo 3. If you play on 360 on Xbox live, add your site url as your motto. Also, you might want to add details in user profiles where they can put their current Halo 2 rating, clan name, gamertag, favorite Halo game, etc.
You should change the skin colors to the theme or Halo (the game), make it nice light colours. Other than that, keep uo the good work.