The REAL benefit of 64bit is the vastly improved speed of computation. Certain hash calculations are OVER 300% (300% = 4 times faster) faster in 64bit! And I benchmarked a 7z compression to be 27% faster in 64bit compared to 32bit. (same machine, installed 32 and 64bit versions, compressed the same files with same settings with each one and timed it).
Er ... incorrect. 64-bit has 2 advantages which are not dependant on the OS: You have twice the registers, and you can (in some cases) do 2 mathematical operations at once, compared to 32-bit. Because of the way modern CPUs are designed, single operations on very large numbers dont really go any faster than before, because the bottleneck there tends to be IO, and that hasnt changed much.
The only problem is, there arent very many applications where this is actually possible. Only specific types of computations really take advantage of this ability to run 2 mathematical operations on different numbers at the same time, and that's mostly the various types of Digital Signal Processing - audio/video (de)compression - and other DSP-like tasks, namely data (de)compression and encryption/decryption, and these are higly dependant on the algorythm used.
This is partly due to compilers (especially on windows, if youre using msvc++, since its fairly horrid and generally lags well behind ICC in compiler evolution), partly due to limitations in the CPUs logic, but mostly down to the reason that makes programming for parallel supercomputers so very hard - its really hard to parallelise serial mathematical tasks.
If you bench Max on a C2D, 32 and 64-bit are almost the same speed. On most things, its within statistical error limits. The gap on AMD processors is a bit larger because differences in architecture penalise 32-bit a little, but that's because of CPU design. Even having twice the registers makes almost no difference on most tasks.
So yes, 7zip will be 25% faster, and so will turning raw video into mpeg4, or compressing a cd into a lossy/lossless codec (though higly dependant on the codec). Everything else however, will be running at the same speed.
If you think this is disappointing, imagine how disappointing it is when you move from a target-compiled 32-bit linux installation to one in 64-bit.
Even though youve compiled all the OS, kernel, drivers and userland for that specific CPU, 99% of tasks will run at the exact same speed, and on top of that, things like flash dont work on 64-bit linux (or at least not easily) 
