I think 40 minutes is an acceptable amount of time, provided you're seated near the front of the plane. There's a pretty good chance you won't be going to the complete opposite side of the airport to switch planes, which will save you even more.

But I'll reiterate what many people have said in that this assumes your incoming plane is on time ... even a 10 minute delay, whether it's in the air or just getting to the gate, could break this connection for you. Make sure at the very least these two flights are on the same ticket so it's easy to be pushed back to the next flight.