Hi, just wanted to add a bit of information on the Audi S8...I have an S8 and previously had an A8L.
Both cars are 100% aluminum engine and chasis. The main difference is in the engine. The standard A8 4.2L is 300-310hp, the S8 is 360hp (in Europe other engines were available in the A8 but in the States we only had the 4.2L V8 or the 6.0L W12). It doesn't sound like much but it's a much quicker and more aggressive car. The S8 engine, while still being a 4.2L has some different components, such as a more aggressive cam and a slightly higher compression. The S8 has larger 4-piston Brembo brakes (not sure if Brembo is standard) that are really superior.
Another big difference is that the S8 is only available in the standard body, not the L (long)...I believe there's about a 4-6 inch difference in the two bodies. The S8 has a considerably stiffer suspension and is lower with larger rubber. As with all S models, the side mirrors on the S8 are chrome. The interior of the S8 has deeper leather bucket seats...the most comfortable of any car I have ever owned. The S8 has Alcantara leather trim (suede-like and extremely expensive as an option on other cars), plus various other minor interior gadgets that are not on the A8. The car is also about 100 lbs (45 kilo) lighter.
Production numbers are a big difference as well. In 2003 Audi produced roughly 21,000 A8's worldwide. These number are a bit inconsistant but from what i've gathered, only 1200 S8's were produced worldwide over its entire 3 year run.
Hope that this helps, sorry if too much information!