Thought I would toss this here. I found it interesting:
"The 4GB is the Process Memory limit. You have been able to run 64+GByte on a Xeon box for years (desktop chipsets tend not to have enough memory slots to go this high). Each process gets its 4GB with either a 2 GB Kernel/2GB user space - or the 1GB Kernel/3GB user space mentioned by the parent.
Since most environments run more than one process, they can take advantage of the extra ram assuming their total amount of allocated space is above 4GB. For that matter, I used to run a 32bit version of BSD 5 years ago that ran on a Dual PIII system with 8GB RAM. Basically we ran 2 caching processes of 4GB each, and some smaller processes that added up to a memory load of 8GB."
What you get with a 64bit operating system is a theoretical 64bit address space for each and every process. In reality different processor architectures offer somewhere between 40 and 48 bits worth of physical address space (Good for almost a Petabyte of RAM). 64bit is really only useful for a few VERY large applications such as Database, a few imaging processing apps, and some massive number crunching... Your average desktop OS application has no need for more than 32 bits, and in fact most of us would actually have slower machines with a 32bit user space.
Further reading seems to indicate x64 will give you access to all memory your system can hold and in an unpaged
fashion. I am definitely trying x64.