The Big 12 promised a balanced and competitive year in the conference’s broad middle class, meaning there would be an exciting race for a possible at-large bid to the BCS and at the very least to get the conference’s spot in the Cotton Bowl. With Kansas State’s win over front-runner Oklahoma, we can add the race for the league championship itself into that mix, and every game among contenders in this league is a big deal. That’s why Texas-Oklahoma State is likely to be the most consequential game of Saturday and where TheSportsNotebook, after looking at the undercard games yesterday, will begin the Friday previews of the three marquee games of college football Week 5…