There are different forms of 64-bit, which provide different things. IA64, which is currently out, is exclusively 64-bit (for now). It runs 32-bit programs very slowly. x86-64, still in development, promises to run 32-bit well, and give good 64-bit performance. However, there are no actual x86-64 processors around yet. It's still an unkown.
BTW, a little known fact is that the assembly language is completely different. A friend of mine (who's a game developer) bitches about IA64, because he doesn't want to spend a year learning a new assembly language.
Sorry I didn't really answer your question, but hopefully I got the ball rolling.
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