They really aren't that impressive. The V8 S4 (B6/B7, 2004+) is 340 hp, but weighs something crazy like 3,800 lbs. Audis generally weight a LOT, my A4 weighs about 3,600, and that's without the stereo and me! They still move quite nimbly for fat cars though.
Now the 2000-2002 S4 (B5) has the 250hp 2.7L bi-turbo V6 and will get 320hp with just a chip. Throw in the next size up turbo (K04, same as the Euro RS4 cars of the time) and you're looking at the 450 hp range on pump gas. Those are the ones to look out for. Or the newer RS4's too. They are also a V8 but with 420hp. Audi V8's sound incredible. Lots of vids on Youtube, no i don't feel like posting any right now. Gimme a break, it's 5am and i'm sippin coffee! :P