You most definetly do not need any more RAM for that machine. Also, a bsd/linux system on there would give you the best performance off the drives, but then the issue becomes getting the TV tuner card to work (though there are cards that work great in linux...if you decide to go this route then do some research on it...there are also linux distros designed specifically for media pc's which make everything really easy, so look for those too, oh, and running the game servers might become a problem).

Also, I think you just shouldn't mix a media center pc with a file and game server. Also, I think you shouldn't even make a file server unless you have some serious leftover disk space, like 200 gb.