Darth Sidious is Palaptine's Sith name - they are the same person. Each Sith is named Darth something - each a name associated with evil (SIDIOUSness, TYRANny, MAULingl and an inVADER).

Link all of the godfathers are DON something, all of the Siths are Darth something.

Might be an interesting thread - What would you call GF characters if they went over to the dark side of the force (like Darth Corleone) or what names would you give the star wars characters if they were in a Mafia gang war - personally, I'd like to hear Yoda do is predicate/subject speaking with an Italian accent (It-faggedabout; sleep with the fishes, you will)