This is interesting.

But I am quite confused about Linux. There are so many distributions and versions. I would not know which one to choose. And after this I am not sure why I should boot it even if it is there. I do not code programs there. My customers all use Windows. Only C++ has some decent GUIs there and games are rare. My modeling tools do not run at Linux, only GIMP and Blender will do. This is very restrictive.

ok, I did some research about memory usage, 32 and 64 bit. This is what I got:

32 bit Windows:
max 2 GB memory per task
max 4 GB memory per OS (some of them gets lost because of addressing the hardware)

64 bit Windows:
max 4 GB memory per task
max 128 GB memory per OS (theoretically it would be much much more)

This shows me that I need much more memory to use a 64 bit OS efficiently. If I have "only" 4 GB then I can subtract 1 GB for Vista and maybe another one (or less) for addressing the hardware. But keep in mind that 64-bit addressing of hardware uses more memory than 32-bit addresses.

At the end you can use only about 2 GB for your task and that is exactly what I get with a 32-bit OS.

So it will only make a difference if I have more than 4 GB RAM in my PC.

lately i switched to a 64bit system using vista with 8GB ram. and aside a hardware defekt and some knowledge issues it runs good.

yes, 8GB and 64-bit make some sense. Even if you lose more through the wider addresses it still has some more RAM left and you can easily run several graphic tools parallel without using temporary memory on HDD.

Maybe I should do it that way:
partition 1: XP 32-bit
partition 2: Vista 64-bit
RAM: 8GB

i can't imagine that wider adresses make much difference in ram usage. if you think of a typical program and how little space pointers take there compared to the data... but maybe i am wrong...

i think installing two windows versions isn't worth the effort. i currently use 64bit XP (with only 4gb of ram) and don't have any problems.

if you need an older windows around for some reason then virtualization (for example with virtualbox or vmware) could be nice.

Yes. I am sure you will not have problems. But as my calculation shows you will barely have an advantage as well. I read several sources about memory usage of 64 bit. And many show that addressing and CPU instructions use about 40 percent more memory.

I also read that the 64-bit version of the game "Crysis" was not faster than 32-bit. They told that it actually was faster but the longer addressing slowed it down at the same time. So it was the same like 32-bit at the end.
And Crysis does not use more than 2 GB RAM (32 and 64-bit).

cpu instructions... that means the machine code and also not the data i think?

i haven't yet noticed any 64bit applications using 40% more ram than their 32bit counterparts.

64bit windows makes a difference for me because of the 2gb per process and 3gb in total limit of 32bit windows. for example 64bit blender can use 3.5gb ram or so on my pc.

and once i upgrade to 8gb i won't have to install a new windows.

yes. When I install more than 4 GB RAM then it can have advantages to use a 64-bit OS. I am sure.

i did a quick test...

32bit blender with a 100000 poly mesh takes 75 megabytes
64bit blender with a 100000 poly mesh takes 95 megabytes

32bit blender with a 400000 poly mesh takes 230 megabytes
64bit blender with a 400000 poly mesh takes 295 megabytes

i wouldn't have thought that but the difference still isn't 40% but about 25%. don't know how it scales though...

You can actually have both Vista and XP by putting 2 hard drives into your computer. I did it with mine after i bought it from dell with Vista installed. If you do what i did, search for VistaBootPRO 3.3, that is what i used to be able to select which OS i wanted to start up when i booted.

Have fun,
Doof