More RAM - Additional Memory Used By Windows
When reserving memory, Windows Vista goes by the amount of RAM installed in the computer. Unlike previous versions, Vista will automatically use more memory for the system if there is more available. The result is that fewer files have to be written to the page file on the slow hard drive, giving the system a noticeable performance boost.
The following diagram clearly shows that Windows Vista will reserve more memory for the system when there is more RAM installed.
We can also see that the 64 bit versions take up quite a bit more memory as well. Again, the explanation is very simple: all of the variables are no longer only 32 bits long, but 64 bits instead. Typically, this makes applications between 20% and 40% larger, which consequently results in a higher memory footprint as well. File formats such as music files or videos are not affected by this.
The upshot is that it doesn't make sense to install a 64 bit version of Vista in order to better utilize 4 GB of memory simply because the 32 bit version would only recognize 3.5 GB. The problem is that while it is true that you would "gain" the missing memory, you would also immediately lose it to the system due to the 64 bit version's larger memory footprint. Thus, using a 64 bit version really only makes sense with larger memory sizes.