I'm close friends with a boy who I've known for about a year (for the sake of clarity, from now on he is A. There's always been a very tiny part of me that is attracted to him, but nothing has ever happened with him because I've never allowed it to happen. When I first met A, I was in the final stretches of a rather miserable relationship, and rather crazily infatuated with his best friend (B). I'd just started college in a city about an hour away from where I used to live, and he and his friend were the only people I knew in the area. Because of that, the three of us became rather good friends.
You need to really evaluate your relationship with C, you say its stressed and that not the same as it used to be. Keep in mind that your relationship with C and your interest in A are two separate things, you don't want to validate your relationship with A based on the deterioration of your relationship with C. Figuring out how its going with C should be your first priority, if C isn't what you really want anymore, and then A is truly what you want, then so be it, but you don't want to have the guilt of leaving C for on a whim because of a fleeting crush with A.
I'm not going to break up with C, because, well, I love him. He's someone I feel comfortable and safe around, and he's been absolutely wonderful to me. I want to know that when it does end we'll still be friends.