what is the difference between model 08G-P4-5173-KR and model 08G-P4-6173-KR?

The 08G-P4-5173-KR has a black shroud and no back plate.  That is the only difference.
 
EDIT: Searching the forums yields results using the part number to include this thread.

^ that is correct. The differences in the shrouds includes the color (obviously), there is no LED lighting around the fans themselves, and the silver pieces on the SC that are screwed in place are molded into the black shroud on the Black Edition. We would not expect you to see any performance differences between these two models and they are able to SLI together.

So the evga 08G-P4-6173-KR comes with a back plate installed?

jim1174
So the evga 08G-P4-6173-KR comes with a back plate installed?

Yes.
 
 
The following link shows you the cards that do and do not come with a back plate on the right side of the chart:   Link