Re: new software if moving to amd64?
Well. Having done a lot of (kindly) support for older machines, i'd say leave the arch as is. While on the other hand it's fun to upgrade to a decent GNU/Linux version (a friend once said Linux is the greatest online adventure ever) so if you are bored, give it a shot :) but that does not imply to change the architecture.
The problem is just the 'fiddling' which consumes your time for nothing. Plus, multiarch (mixed i366 and amd64 packages) in reality is not that easily done as said. In my case, when i tried to set up google earth 32 bit (in a native 64bit env), it finally ends up to move most (or all?) of the installation to 32 bit. Well, it still works, but at least looks ugly in the package manager :) and i'm sure the respective package dependencies could be resolved differently, but i won't blame the maintainers. It's just a lot of work, on all ends, to make it real, and the tiny word 'work' resembles the huge problem, paid or not.
There is one feature of Windows 7 that struck me though i did not really dive into it ... when a software does not launch or work properly, the Windows OS tries to identify and solve the problem, for example by switching to 'Windows XP mode' where 32 bit apps run in an emulation environment. Maybe they chose the safe bet, i can't tell, especially since Windows always makes a big fuzz over solutions to problems that should not exist in the first place.
Our friendly competitor have a not so bad overview for beginners on these pages
There are very few GNU/Linux apps which in the past were available only in 32 bit packages, proprietary things like google-earth and maybe skype, meaning you'd need the debian multiarch feature. Meanwhile those issues could or should be solved, but still, there's no guarantee since those the props usually do not have significant GNU/Linux support on their end. Yet i'm running googleearth right now and the debian maintainer even tries to upgrade to ge7.
As for the gain of 64 vs 32 bit, one should ask how urgent are high performance tasks on the respective computer. This is not totally trivial even if you're only doing some office and internet, because most people at least want smooth movies and a graphical desktop. Besides the CPU, also the GPU is involved. I imagine that typical GPU tasks involve massive parallel computing and other stuff that possibly could profit from 64bit. Please don't nail me down on this, i really don't know much about, but i wonder if the 32/64 bit question is just the same for GPUs. Maybe someone can enlighten me.
ps. It seems there is http://www.nvidia.com/object/feature_64-bit.html