Currently, there is not an S6 sold in America, nor an S8. There were "S" trim vehicles for the A6 and A8, but not since the redesign for either model have they been sold here. But, Audi will soon be importing the S6 to our shores, and will soon make an S8 with rumours of the V10 out of the Gallardo, and, lastly, we'll be getting an S3 finally as well, a 2-door AWD version of the great A3 sold here.
On almost all Audis (except the TT), you can get an "S-line" trim package, which generally is little more then visual upgrades, sometimes wheels, mild body extensions, special belt-line trim or interior trims, but none of the performance that makes S versions so good.