There still seems to be a bit of confusion over the identity of 64-bit processors for Windows operating systems. Windows runs on two types of 64-bit processor, identified as x64 and IA-64.
Going with the oldest first, IA-64 stands for Intel Architecture, 64-bit and was Intel’s attempt to move to a 64-bit architecture. It remains compatible with 32-bit user-mode code through hardware emulation, but this hardware emulation performs so poorly relative to a genuine x86 that all operating systems shipping for IA-64 now include a software x86 emulation called IA-32 EL. IA-64 is the instruction set; the family of processors which implement it are named Itanium (which you’ll see critics refer to as Itanic, suggesting that it’s sinking) and so you’ll sometimes see code written for them referenced as Itanium Processor Family or IPF. You need a new operating system to run any code on an Itanium – it cannot boot a 32-bit operating system.
x64 is actually a Microsoft term and stands for Extended 64. It is implemented by both AMD and Intel who respectively call it AMD64 and EM-64T (Extended Memory 64 Technology). AMD invented it as extensions to the existing x86 architecture. An x64-capable processor natively implements x86 in hardware – it is an x86 processor, with additional modes. It boots up in the same 16-bit ‘real mode’ that an x86 does. It can run existing 32-bit operating systems. You may well have an x64-capable processor without realising it. All AMD Athlon 64 and Opteron processors, and Intel Pentium 4, Pentium D and Xeon processors built within about the last year, implement x64. To check whether your Intel processor supports x64, use Intel’s Processor Identification Utility.
Itanium had the early lead and for a while held the general performance crown, but the relentless increase in x86 clock speeds eventually had AMD’s Opteron overtake it for integer calculation performance. Itanium still leads for floating point performance but has been stuck at 1.6GHz for about two years, if I recall correctly. It’s liable to be beaten by Intel’s own ‘Woodcrest’ Xeon-replacement later this year, in my opinion, if it remains stuck at this (now relatively low) speed.
Itanium is now pretty-much reserved to big-iron datacentre servers. It’s good for highly computationally-intensive applications. If you just need lots of memory, go with x64. Itanium used to have an advantage in number of supported processors too, but IBM recently started selling the xSeries 460 which supports up to 32 dual-core Xeons. This hits Microsoft’s limit of 64 logical processors which applies to both architectures.