Good luck to you. I think this is a little too late to bank on a niche like Mass Effect 3. If I were you, I would have launched ME3Board months ago.
Why? Earlier is always better for niches like Mass Effect 3, or Call of Duty, or Battlefield. And you missed the boat on the controversy surrounding the ME3 ending. That would push activity on your forums a lot faster than launching it right now.
Trust me on this one, because I launched similar to you; I launched MW2Forums/MW2Boards later in MW2's lifecycle.
The other thing I'd like for you to understand is that "Board" or "Boards" aren't keyword heavy.
So, you have to target the google search engines squarely on the [Mass Effect] keywords alone.
You will have to spend a lot of time making threads, picking out subjects that interest not just you, but also your members. And you WILL have to do some heavy marketing in order for that site to succeed. It will take a lot of hard work.