EM64T is targeted to 64-bit operating systems. Period. If you’d like to buy a 64-bit Celeron D or Pentium 4 for when Windows 64 and native 64-bit software reach the market, go ahead. But keep in mind that you won’t be able to use EM64T exclusive features unless you run a 64-bit operating system AND 64-bit software.

If you have a 64-bit Celeron D or Pentium 4 and Windows 64, 32-bit software will run just fine, however it will run under compatibility mode, meaning they will “see” the CPU as a regular Intel IA32 engine. If you use “heavy” applications and are thinking of moving to 64-bit computing to have more than 4 GB RAM available, keep in mind that you will need new 64-bit version of your software, or they will still access only 4 GB RAM, thus not solving your problem.

Also keep in mind that the external address bus of EM64T-based CPUs isn’t 64-bit wide, so no Intel CPU using this technology can access 16 EB (exabytes) of RAM (2^64), as you may think. The maximum amount of RAM memory a CPU can access under 64-bit mode depends on its implementation. 64-bit Celeron D, Pentium 4 and Xeon CPUs can access up to 64 GB of RAM while 64-bit Xeon DP can access up to 1 TB of RAM. Once again, keep in mind that under 32-bit mode or 64-bit compatibility mode, the CPU accesses only 4 GB of RAM, even if it is a “64-bit CPU”.