Ars Technica explains 64-bit x86-64
Ronald already covered this in the Shortbread, but PYAITK goodness like this deserves a full post. Ars Technica has written an excellent introduction to 64-bit computing and the x86-64 architecture that even Intel fanboys should go read from start to finish.
Since every enthusiast should head over to Ars and read through the entire article, I'll just give you the shortest of teasers here:
The present article outlines what AMD hopes is the next step in x86's evolution: x86-64. As we'll see, x86-64 is more than just a 64-bit extension to the 32-bit x86 ISA; it adds some new features, as well, while getting rid of some obsolete ones.
The article might be a little too technical to tackle first-thing in the morning, so you might want until after at least your first cup of coffee.