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.