In my experience, Big Spaceship is great (they design sites for Hollywood flicks) and Big Noise does great work too (check out Billy Gilman's official website at: http://www.billygilman.com/)!! I've done some web design for local artists and whatnot, but I'm not on those guys' level yet (Big Spaceship and Big Noise)... but hopefully one of these days!
I personally wouldn't recommend using PHP for a live chat program, but, I have not personally used PHP for a live chat system. In my experience, Java is better suited for this task. Hotscripts.com has many Java applets and servlets - and I'm sure there's one that would suit your site well. Best of luck in finding one that suits your needs!