The important numbers are:

RIM Markings: 622X17
Tire Markings: 42-622

Everything else is marketing gobble gook.

I would not put anything wider than your current tires on those rims.

As noted by Andrew; as narrow as 25mm would work.

As Bill noted: "It's the other 25% that you need to worry about." Tires that are the best on the road are practically unrideable on gravel and rougher stuff.

Check out some "cross" type tires in a ~32mm width to cover that "other".