Have a look at this Wikipedia page which outlines some differences between the AMD and Intel versions of 64-bit :
http://en.wikipedia.org/wiki/EM64T
It isn't only Postgres. I work on a number of other server applications that also run much faster on Opterons than the published benchmark figures would suggest they should. They're all compiled with gcc4, so possibly there's a compiler issue. I don't run Windows on any of our Opteron boxes so I can't easily compare using the MS compiler.
