I don't know how to explain this "excitement" myself, except through examples that might bore you because I don't know the details of how to write a program that takes advantage of a 64 bit cpu. The excitement is mainly about speed, I would say. To make compiled software harness that speed, it may be that only a compiler flag has to be turned on; I'm not real sure and should research it. But here are some examples:
64-bit code, in general, tends to be a little slower than 32-bit code on the same CPU. There are exceptions, of course.
AMD64 has the advantage of adding a number of new registers, and that can boost performance enough to make up for it. AMD claims something like a 5% net increase with AMD64 vs. IA32 on their own chips. This is not enough to really be visible. Unless you happen to have one of those apps that's really well suited to AMD64, you won't benefit much.
Where the Athlon 64 and Opteron really shine is the integrated memory controller. Without it, early Opterons and A64s would have been slower than the Athlon XPs that were released before them, according to AMD's own performance numbers.
Also keep in mind that many of the most demanding apps (e.g., games and workstation graphics) are heavy on floating point math. Those won't see any benefit, and many companies have struggled to even match the performance of 32-bit code, much less exceed it. The Unreal Engine (used in numerous games) is one notorious example.
In answer to your question, the compiler will automatically attempt to make use of the extra registers when compiling on your AM64 system. Just don't expect too much.
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