The difference between AMD's upcomming 64-bit processors, code-named Clawhammer (home) and Sledgehammer (server), final names Athlon and Opteron respectively, and Intel's Itanium and Itanium 2 is that the Intel processors have a completely new instruction set that requires a recompile of every program (Itaniums are just for high end servers though).
On the other hand, the "hammers" use what AMD calls "x86-64" (
www.x86-64.org), an extension of the x86 used by today's processors, adding 64-bit registers and memory access. That way, they still run 32-bit applications, and can run them concurrently with 64-bit ones.
However, in order to run 64-bit apps you do need a 64-bit operating system. Microsoft has announced they will deploy a x86-64 version of Windows XP, and several linux distros are also going to make x86-64 versions.
It is said that Intel is preparing another 64-bit processir, aimed at the desktop market, that will run 32 and 64 bit apps, much like the hammers. The problem is that if AMD's chips are a huge success then Intel will have to adopt x86-64 for their processor, in order to avoid two competing 64-bit standards (not counting Itanium's own standard), which will mean paying money to AMD to license x86-64. Not likely
About what apps will be ported to 64-bit, well, I'd say that nearly every open-source app will be ported, especially CPU intensive ones like LAME. It would mean a huge gain in performance. Small proprietary programs from small companies will probably be in the front line too, again especially if they are CPU intensive. As to big programs by big companies, they will most likely take some more time, though I think when the giants see the benefits of 64-bit they will deploy 64-bit versions with their next product line.
Media Jukebox should *definetely* make a 64-bit version, as any media application should. It will make the app faster and free up more CPU for other programs (most people listen to music in the background while running other programs). You should check x86-64.org once in a while to see if your code is easily portable
Just my thoughts