How many build servers are actually constrained by CPU and not IO?
If your build times are in excess of a few minutes it's probably better to distribute the build across many machines rather than trying to build one server to rule them all.
As for benchmarks there are extensive reviews online of the bulldozer branded as "Opteron".
In my experience (game development, hundreds of thousands of LOCs of C++), compiling is purely CPU limited if your HDD isn't utterly braindead (e.g. at or above normal desktop 7200 rpm levels).
If your build times are in excess of a few minutes it's probably better to distribute the build across many machines rather than trying to build one server to rule them all.
As for benchmarks there are extensive reviews online of the bulldozer branded as "Opteron".
http://www.phoronix.com/scan.php?page=article&item=amd_b...