Shimano America XT RD-M770/M771 Rear Derailleur

The GS is a mid cage model and is designed to work with up to a 33 teeth rear cassette. The SGS is the long cage model and is designed to work with up to a 42 teeth rear cassette.If you're new to MTB and shifting, go with the long cage as you'll probably cross-shift a lot. Otherwise go with the mid-cage which is what most people run IMHO.

The GS is a mid cage model and is designed to work with up to a 33 teeth rear cassette. The SGS is the long cage model and is designed to work with up to a 42 teeth rear cassette.If you're new to MTB and shifting, go with the long cage as you'll probably cross-shift a lot. Otherwise go with the mid-cage which is what most people run IMHO.

Top normal is most common. Derailers are defined by the normal position of the derailer (that is the resting position), either the top gear or the lowest gear. The way to check a derailer is to release the shifter cable and see where the derailer moves to. The top gear is the small side fo the cassette and the low gear is the larger side of the cassette.

Simple answer: yes.It's a loaded question though. While this derailleur will work just fine on an 8 speed setup, an 8 speed chain is slightly wider than a 9 speed chain, which can grind the cage pulleys down if you do nasty things like cross-chaining.Beyond that, most 8 speed systems aren't worth upgrading to something this fancy-schmancy. If you're wanting to move on up in the componentry world, be sure to grab all of these things too:http://www.backcountry.com/store/SHI0042/Shimano-America-XT-9-Speed-Bicycle-Chain.htmlhttp://www.backcountry.com/store/SHI0043/Shimano-America-XT-9-Speed-Cassette.htmlhttp://www.backcountry.com/store/SHI0034/Shimano-America-XT-Rapidfire-Plus-Shifters.htmlIf you're not down with all of that, seriously, convert to SRAM X-9. It's better, lighter, and cheaper.