Is there better glass for DX?

Is there better Af-S glass for our DX cameras? Good, better, best? Or is that availability just for FX? I understand there will be certain lenses that are a bit better than others, but in doing my research I can't find anything; for example G = good, B = better, A = best glass. It seems DX has the glass it has and that's that.

The faster glass tends to be better. The Dx 17-55 2.8 is considered "pro" in the Dx lineup. You might (would in my opinion) be better off buying Fx glass and compensating for the crop factor. By using Fx glass you are using the center of the lens and getting the best quality. This also covers you if you move up to (very carefully not saying "upgrade") an Fx body.

But are the lenses AF-S (focusing motor in the lens for the D5100) or will using an FX lens make the camera manual focus from then on?

Moab Man said:

But are the lenses AF-S (focusing motor in the lens for the D5100) or is using an an FX lens make the camera manual focus from then on?

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All newer FX lenses are also AF-S. As long as the lens is AF-S you'll have auto focus on the D5100.

As John states, FX and AF-S are separate designations. FX replicates the 35mm format, so it is compatible with many older AF and manual lenses. But given that pros rely on FX formats (usually!!) there have been AF-S lenses in that format for a while.