There are basically two options available for the RS4
RS4 DUAL SYSTEM, B5 2000 – 2002, CAT back
RS4 HI-FLOW CAT ASSEMBLY, B5 2000 – 2002 Full system
Both have the Pulse twin exhaust pipes
Basically one is Cats back and will just give a louder sound of exhaust note, sweet

The second is the full system with new CATs, this gives a much higher gas flow and will ultimately improve performance up to 458 as stated by MTM, similar for AmD etc. However if you have the full system it will need a remap to ensure fueling is correct and so avoid mechanical failure. But this is a nice upgrade and IMHO is what the car should have been built to. Excellent

Hope this helps, any other comments anyone?
Also just a thought they have made some different CATs, 100 cell and 200 cell which refers to the density in the exhaust, the less dense the higher the gas flow so performace is increased, as is the noise!
I believe the OEM Cats are 400 cell but of a different material, hence the 200 cell still give the same emmisions
Full Stainless Steel Exhaust System (CAT back), £680 Supplied, £750 Fitted
Stainless Steel Sport CAT's & Downpipes £1140 Supplied, £1450 Fitted
Cheers
Bushy