they are both 32-Valve heads. The S8 has more aggressive cams, a variable intake manifold and some other tricks to extract those 40 more horsepower from the essentially same engine block as in the "regular" A8. For exackt specifications, you night want to have a look at the Bentley manual .
Torque ratings remain pretty much unchanged with the S8 engines; they are all about more power. If you are after gobs of low-end torque, you`d need to look at forced induction.
There is one guy who`s done what you have in mind. If you speak german, check out his web page, also the twin turbo project