17" = 43cm.

Your bike: 42.4cm

Newer model: 45cm

Depends also on the amount of weight you intend to carry. If you are going to be fully loaded (40+ lbs) you would want as long as possible -- 45-46cm would be best. Bikes with this CS length are generally very good for fully loaded touring. Many of the "sport touring" models have CS lengths around 43cm, and they are usually intended to carry about 25 lbs, which is about the minimum weight for totally self-supported touring with tent, sleeping bag, stove, tool kit, etc.

Basically you want the rear load as far forward relative to the rear axle as possible without having heal strike, so larger loads generally require longer CS. Of course, the shape of your panniers is also a factor. If you haven't got your panniers yet, you could look for a pair with the shortest front-to-rear dimension while still having the volume you want.