Abbas Jaffarali compares the 64-bit P4 and Athlon64 at Tbreak.com.
Quotes:
"We’ve taken some of the benchmarks we commonly use in our CPU testing and ran them across Windows XP in 64 bit. Keep in mind that these are 32 bit apps that are running on 64 bit XP under WoW64."
"... Windows and the drivers used in this testing are in their beta release. The numbers are there for you to see where the CPUs stand as of now although they are going to get better as both the OS and drivers are optimized."

Microsoft is expected to release a set of benchmark data shortly after the general availability of the Windows OS for the x64 platforms. Due to the compatible nature of Intel EM64T and AMD64 there's only one binary necessary to support both architectures (OS and applications).

CPU data using Linux.