The Astros certainly have the starting depth to run 4 different pitchers out there. The critical innings for Clemens in my opinion are the 1st inning and the 6th. The 1st inning, because over the years, he's not into his full rhythm early (He's not the only great strikeout pitcher with that tendency). The 6th inning because this season, that is when he typically starts to struggle with command and his pitch count/batter increases.
Pet te is a good choice for Game 2. He pitched Game 2 of the 2003 WS for the Yankees and he was 1 batter shy of pitching a complete game (W, 6-1). Technically, Garner could go with Oswalt in Game 2, since Roy would have 3 days rest, but Pet te is more familiar with pitching in US Cellular in an American League setting.
I like the Astros' options just the way it is, right now. Two starters with lots of AL experience pitching in Chicago against DHs. The 2 young NL Astros' "system" pitchers opening up at Minute Maid Park. Backe and Clemens have the better batting averages of the 4 (nothing to get excited about though) and they would project to be hitting in Games 4 and 5.
Strategy changes with injuries or poor starts, so all of that could go out the window, but as it stands now...the Astros are hot and hey, Wild Card teams seem to be the trend on WS winners lately!