Here is the deal :
"first desktop version of Windows to support 64-bit processing chips, which can access bigger chunks of memory and move data around faster than 32-bit chips in wide use on PCs since the 1980s."
--in English: A 64 bit AMD chip running version of 32bit windows is not fully taking advantage of the 64Bits. This is why microsoft is rewriting Windows to take advantage of the new AMD 64 bit chips.
Some people were under the impression that just because you have a 64bit processor running regular old windows, you were getting full performance.
What does "full performance" mean? 64-bit Office is not inherently faster than 32-bit Office, all else being equal.
The world went through the same discussions and confusions 10 years ago when we went from 16-bit to 32. And I've got a dollar the same thing happens 10 years from now when we go to 128.