Talk me about 64 bit systems...

Suppose you just buy a new piece of computer, with 64 bit technology and everything you consider cool. And suppose you'd like to install a GNU/Linux system on it.

As you've got a 64 bit architecture, I think you should use a 64 bit distribution, right? Once you installed it, since the 32 bit instruction set should be still available, you should be able to run 32 bit applications as well.

The only thing that may prevent you to run it would be the use of shared libraries, since a 32 bit application could never dinamically load a 64 bit shared library...

Is there a leak in my reasoning?

Consider that most programs don't need an extra 32 bits for their computations (i.e. they work with values usually less than 4 billion), and that 64-bit code takes twice as much space for each immediate value (the upper 4 bytes always being 0), and you'll see that 64-bit is more of a waste. The only way I see it would be useful is if you need more than 64GB of RAM and/or 64-bit integers are needed in your programs -- enterprise or scientific computing applications are some examples of this.