Atlanta is considered an up and coming team. Lots of weapons on offense, but they shouldn't be any scarier to the Niner's D than Arizona or Minnesota, especially with the Niners playing at home.
But it will still be a big test. Atlanta has had 2 weeks to prepare. The Niners will have to continue containing the running game and pressuring the QB. Defending Gonzalez will be big. So far the D has been able to play a lot of zone but with Gonzalez working the soft spots in the middle it could be a long afternoon if there is no pressure on Ryan.
The real question, though, is how the offense performs. Atlanta's D doesn't have a rep and has not been that good this season. Overall the D is very young. The Niners will need to get the running game going to put up enough points and to keep the ball away from Atlanta's offense.
Should be a great game, and a win will be a big boost to the program. A loss won't be the end of the world, they will still be on track for double digit wins, but they will lose a lot of momentum.