More on Rear Derailers - Shimano Names, Compatibility

Also from a thread on MTBR.com:

"Most rear derailleurs are made the way they have always been, in what Shimano now calls a 'top-normal' configuration. What this means is that if you disconnect the cable, the derailleur returns to the smallest cog, which is the top gear - it's 'normal' state is the top gear, hence 'top-normal.' They wish that we would all buy 'low-normal' derailleurs, which is exactly the opposite - the normal state is the biggest cog, or the lowest gear. So now we have two different models, which in the LX world are M580 (low-normal) and M581 (top-normal).

Then there's cage length, which is confusing enough to the majority of people that it is a sticky at the top of this forum. Basically longer cages allow you to run bigger gear spreads witout fear of cross-chaining in exchange for slower shifting and higher weight. Medium cage derailleurs (the only truly short cages are on road bikes) are far less forgiving of cross-chaining but are lighter and shift quicker. The majority of people in this world use long-cage derailleurs and are thrilled with them. So Shimano makes it's nicer derailleurs in long cage (SGS) and medium cage (GS) flavors for those who want the choice.

So, to end this mini-novel, an LX derailleur comes in 4 flavors:
M580 GS Low-Normal medium cage
M580 SGS Low-Normal long cage
M581 GS Top-Normal medium cage
M581 SGS Top-Normal Long cage. This is the one you should probably get, and in years past this was the only choice you would have ever had."

Also, on SRAM-Shimano compatibility:

• All Shimano shifters are compatible with SRAM front derailers; all SRAM shifters are compatible with Shimano front derailers.
• SRAM X.x shifters require SRAM X.x rear derailers.
• However, SRAM Attack and Rocket shifters are compatible with Shimano rear derailers.