quoted from https://answers.yahoo.com/question/index?qid=20060928075708AAs3Jdi
ia64 was the first 64-bit processor on the market, in the form of the Intel Itanium series. It is designed to be strictly a 64-bit processor, and has a completely different code structure than an x86 or x64 series processor. It can run 32-bit code, but it has to emulate to do that so there is a major performance hit. Intel thought that the move to 64-bit would be best accomplished with a completely new type of processor, dedicated to 64-bit computing. Unfortunately they guessed wrong - with the massive amount of 32-bit applications already in the field, and the poor 32-bit performance of the Itanium, it hasn't really taken off, though it is still out there.
In order to run on an Itanium processor, you need a version of Windows
or another OS written specifically for it - a standard copy of Windows
Server is incompatible with that processor.
On the other hand, AMD came out with the Athlon 64 series processors.
These were x86 compatible, meaning they could run 32-bit code without
any performance hit, and the 64-bit portion of the processor was simply
an extension of that. They were dubbed x64 to indicate they were
x86-processor compatible. Intel is still playing catch-up, though a good
number of their processors today do support 64-bit computing, not all
of them do. And they tend to have more compatibility problems - AMD
processors are actually more compliant with the 64-bit standard than
Intel.
The bottom line is, unless you are running a server with a specific need
for an ia64 processor, you will want to use an x64 processor, and at
least right now, I would recommend AMD. Windows XP 64-bit Edition runs
very nicely, and it runs most 32-bit programs without any problems. Note
though that with Windows x64, you can't run any 16-bit programs...