Architecturly speaking the The Intel EMT64 and the AMD 64bit proscessor are faster than X86 based P4s and X86 AMD athalons not because they are 64 bit but because they are more efficent design.
64 bit processors are faster for the following reasons.
1. They use what is called a Hyper Transport Technology.
http://www.amd.com/us-en/Processors/ProductInformation/0,,30_118_8826_8834~14902,00.html
2. They have an Integrated DDR memory controler.
http://www.amd.com/us-en/Processors/ProductInformation/0,,30_118_4699_7980^7983,00.html
3. Both Intel EMT64 and AMD 64 BIT CPUs have this extended memory technology so this isn't a dispute over which is better Intel vs AMD war.
Plus AMD and Intel will have dual core 64 bit chips here very soon. AMD allready has them. This is two cpus in one. Basically what im trying to tell you is P4s and X86 CPUS in general are obsolete.
The P4 has hit its limit and will unfortunatly never run any faster than what the fastest P4 is running right now. The future is bleek for X86 computing to be honest with you both a EMT64 OR an Atholon FX57 will out perform any P4 or AMD X86 Cpu to date.