I found this article about MLB's tie-breakers. It's a bit complex but in a nutshell if the Cards, Astros and Reds finished tied at season's end then there would be a mini-tournament to determine the NL Central winner. The three teams would be seeded according to their record against each other. I believe this would give the Reds the advantage as the Cards and Astros would play game one (Monday) and the winner would play the Reds the following day (Tuesday) to determine the division winner.
Sad thing is that even if this occurred Harang is scheduled to pitch Saturday and Arroyo Sunday. Who would start the most important game of the season?