I am planning several trips - and want to try and get more on the 777, 757 , or 737-800s ( soon the 900 ) how can I find out easily what planes fly a route - or is there a search engine that allws you to select a flight based on the aircraft flown?

I know that the airline reserves the right to change planes ( ie 777 switched to a DC10, etc. ) but those are the breaks :P