Our primary goals in this article are to shed some light on the performance differences between the current AMD and Intel platforms in a heavily multithreaded server environment, and to see how well the platforms scale. Since the systems we used were on loan from a local dealer we had limited time for testing, and could not test every type of applications. One application we did look at is ESX Server; this is an important topic these days, as VMware's operating system can bring any server to its knees if you run many logical servers on it as virtual machines.You can check it out over here.
AMD Opteron vs Intel Xeon server performance

Tom's Hardware Guide compares Intel's with AMD's server processors: