On the official SW site, they list The Sith as an "organization" rather than a "species."
The original Sith, if you will are described thus:
"Long before the Republic rose, there lived a culture on the planet Korriban. These primitive people were called the Sith, and the Force flowed strongly through their bloodlines. Although they didn't practice the Force as the Jedi would, they were talented in their own brand of magic."
After the naughty Jedi who wanted to dabble in the Dark Side were cast out and settled on Korriban:
"Millennia of interbreeding blurred the distinction between Sith native and offworlder, and the term Sith came to encompass not only the indigenous people of Korriban, but also the powerful overlords that ruled them."
So, the original Sith must be capable of producing offspring with humans/Jedi; I would guess that, from the description, the original Sith were mostly human-like.
After all, if they were weirdo bug-things, shouldn't Palpatine have antennae?
EDIT: That's the only way I can wrap my brain around the idea of "True Sith." The idea of this other type of evil badguys who are the "True Sith" being out there, beyond Republic space, seems to go beyond anything remotely canon and also against what my childhood soaked up about the Star Wars universe.