I wonder if I am going to have to collect my luggage in Atlanta and go through customs there, and if so, how much connection time I need. I have an hour and 40 minutes: enough time?

If Atlanta is your port of entry into the US, then yes, you'll go through passport control and customs there.

If your flights are all on one ticket, then yes, it's enough time - the airlines can't sell tickets that violate Minimum Connection Times (MCT) published for various types of transfers, e.g. international-to-domestic, domestic-to-domestic etc.

Thanks, Gardyloo. Your second paragraph is especially informative. I have occasionally avoided flights with what seems like insufficient connection time. I guess I worry about flights being late since they often are.

Yes, they often are late but in case the incoming flight is delayed then the airline will try to adjust your reservation on the next flight to SFO, but according to seat availbility !!.

It may be enough, but what if your first flight is late?

By the way, if you have less then one hour, they will let you in ahead of the line. I saw it happened in Atlanta.

That's exactly my concern, Dayenu: that the first flight is late. I'll just have to take my chances. The worst that can happen is missing the flight and catching the next one.

It won't be a disaster, after all !!! That's not a route that has only 1 flight/day or just one or two/week !! Enjoy your trip !