The nice people of Tomshardware site have made a great article about the benifits and the drawbacks of large system ram and all you need to know in regards of it, you can read it here


Most users will be sobered when they equip their systems with 4 GB of RAM and find that only 3 GB is recognized by the BIOS and by Windows. This is neither a bug nor due to any hardware error. The explanation is simple: 32-bit systems can only address up to 4 GB of memory. Additionally, many add-in cards and on-board controllers require memory addresses in order to be accessible. This is called "Memory Mapped IO" (MMIO). Since this memory range has to lie within the 4 GB, it is subtracted from the installed and available RAM.

Thanks to a technique called memory remapping, it is possible to move around parts of the system memory in such a way that the full 4 GB is still available for use. The trouble is that this feature had to be deactivated in Windows Vista due to compatibility issues.
In order to be able to utilize the entire system memory, you will therefore need to use a 64-bit version of Windows Vista. In this article, we will take a look at memory usage under the 32-bit and 64-bit versions of Windows Vista, and analyze how the operating system behaves with different amounts of RAM.
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.

Read more at Tom's Hardware site