Adobe Audition 2 requires a processor that supports the SSE instructions set, and there are some questions on exactly what chips support it. On the Intel side it's easy: any Pentium III or later and you're ok, including the Pentium M, Xeon, and Itanium. On the AMD side it's a little trickier. AMD first introduced support for the SSE set part way through the Athlon line, which means that Athlon processors at the produced early in the cycle don't support it but later ones do. If you have an Athlon XP or later you're set.
If you have any questions about whether your processor supports SSE, there's an adobe techdoc available that includes a link to a utility that can tell you for sure. (The doc references Premiere Pro but applies equally to Audition.)
Now, some folks may wonder why we decided to require SSE. Really, it just boils down to the fact that we were able to get some pretty significant speed increases in many parts of the program by using it and it would have been very expensive engineering-wise to provide a complete set of code that didn't require SSE for folks who had older chips.
There's also a question floating around regarding AMD support in general. Audition 2 works well on AMD processors, and we have a bunch of AMD-based machines here in the office that we test on regularly. We did use some Intel-optimized code for our FFT (Fast Fourier Transform) calculations which helps accelerate parts of the program (the spectral views, other analysis tools, and many effects) even further when running on Intel chips. And why did we use this Intel-optimized code? Intel provided us highly optimized libraries that have a bunch of developer-years of work in them, as well as the assistance of several engineers to help us integrate the work, so we were able to get a performance increase for many of our users without impacting the number of features our core developers were able to develop. We didn't find any disadvantage of using this code on AMD chips, but there was a good boost for folks with Intel chips, so the choice was easy.