The vcvarsall.bat that does setup de build environment let you choose between x86 | ia64 | amd64 | x86_amd64 | x86_ia64.
First option is obvious for 32bit, but the question is: what would be the optimal option between the other 4 if you want to build for 64bit and you want to run on all 64 bit machines.
Let say that you don't have time/resources to make 4 builds for 64 and you have to choose only one of them?
I found a page that clarifies this issue http://msdn.microsoft.com/en-us/library/x4d2c09s%28VS.80%29.aspx
x86x86_amd64amd64And you can ignore the other options because Itanium does not have a real market share.