Wrong architecture, I'm afraid. ia64 is for the Intel Itanium - which
is a rare beast on it's own and incompatible with everything else.
Unless you're very lucky, or you're a major clearing bank needing to
chunk vast amounts of data, it's almost certain that you'll never
come across one in the wild :)
Everything AMD/Intel that is 64 bit capable uses the amd64 .iso.
This includes later model Sempron/Turion/Opteron and, of course, AMD64
and Intel Xeon/Core Duo/Core2 Duo. AMD got there first, so in Debian
at least, it's amd64 - you may see people reference x86-64 as well.
The Debian distro is purely 64 bit at the moment: you can install some
32 bit libraries for backward compatibility. To get optimum
performance from some legacy 32 bit (particularly Adobe Flash and
some other multimedia) some people run these in a chroot 32 bit
environment.