The dual mac pros people are using for music production and video encoding aren't server, I don't think.
And the clock speeds for the opterons are low, especially when the cores start getting larger.
Dual Mac Pros use Xeon server chips. If you want to build a dual cpu machine, Xeon and Opteron are your only alternatives. Regular non-server chips do not have the circuitry necessary to work in a dual configuration. As far as the lower clock speeds, a lot depends on your software's ability to utilize multiple cores effectively. A good example would be 3D rendering using a bucket renderer. Since they can render each bucket as a separate process with a dedicated core, a 12 or 24 core machine will will outperform a 4 or 6 core machine running at a higher clock speed.