AMD is currently better in most apps, period!!! Yes dual core will help with photoshop and Autocad....I am less familiar with audio editing....
In autocadd dual core will help in rendering, and some other functions....
What kind of budget??? cause price points especially on the extreme budget can make som eof the low end 8xx series a possibility...
Otherwise I say get a 3800+ X2, opteron 165 or 170....All reasonable price...Most A64 sckt 939 mobos are quite reasonable in price as well...even quality (non ECS bundles) boards...