Xeon is vastly better than an AMD 64; and they're not against Opterons, they're all on their own. Opterons are Server Processors; the many people will say they're Xeon's AMD version, but Xeon is a Server/Workstation Processor and not meant for High End. Opterons are cheap All Server Chips, Xeon's are ever cheaper Low-End Processors for Servers and Workstations and Powerful Computers. Opteron are against Itanium2; and a vastly out gunned in the respect.
I'm sure anybody here knows AMD64 to Opteron Benchmarks; so compare percentage to this: Xeon Benchmarks from Hexus
. They're a pretty good site, even if they are named after the evil thing from Fern Gully.
High End Computers are Xeon
But they're not cheap for it. A good one, the one in that Test, will set you back all of £2,000 (Dual Core) if you want to put a reasonable graphics card and such like in a basic setup you'd get from HP
Building Seperatly would set you back as much. But try and find an Opteron like that!