This is the press release from Valve announcing "immediate availability of a 64-bit version of the Counter-Strike dedicated server using the upcoming AMD Opteron™ processor," though where this is available is not clear. Word is: "In a straight port of code highly optimized for x86-32, Counter-Strike dedicated server tests with both 32- and 64-bit versions revealed a 30% clock-for-clock gain, and is expected to show further performance gains in future upgrades."