Next we set up a fresh install of the latest Windows XP Pro x64 Edition RC1 build to test out Intel's EM64T extensions in any scenario we could possibly exercise them in. Unfortunately there are very few 64-bit applications or benchmarks on the market, but SiSoft Sandra is one of them with an x86-64 install version available.

SiSoft Sandra - 64-bit Edition Synthetic Testing Performance Windows XP Pro x64

Pentium 4 Extreme Edition 3.73GHz - 64-bit Tests
CPU, Multimedia and Memory Benchmarks

Reference Pentium 4 Extreme Edition 3.73GHz - 32-bit Tests
CPU, Multimedia and Memory Benchmarks

Sandra's CPU and Multimedia tests show measurable performance gains with the Pentium 4 running in 64-bit mode with Windows XP Pro x64, even though the OS and associated drivers are still very much in beta stage from Microsoft. However, system Memory performance is not really affected either way.