Bush and McCain carried 54 congressional districts that sent Democrats to the House in 06 and 08. The Republicans need to flip 41 of them to retake the majority.
Charlie Cook, Democrat pollster, concedes 20 are already gone. Halfway there.
The other 20 are there for the taking, provided that the GOP properly recruits, funds and supports good conservative candidates.
Vets returning from the War on Terror would be a great place to start.
A interesting thing about the 2006 election, many of the successful dems ran as fiscal conservatives when compared to the Rino Republicans they ran against. At least that is what I read at the time.