I'd like to point out that the Sedan and Wagon fenders are different. Also you need a matching front bumper cover, as it's made to conform to the appropriate fender.
All 04-05 Impreza Sedan fenders are the same (doesn't matter if they are off an RS, WRX, or STi)
All 04-05 Impreza Wagon fenders are the same. (Outback Sport, RS, or WRX)