OCModShop posted a guide about using 4GB of RAM with Windows Vista:

All 32-bit Operating Systems can only physically address 4 gigabytes of RAM, due to the math involved (232 = 4,294,967,296). Depending on your hardware, Vista may only see 3GB or 3.5GB of the total RAM installed, because there are some devices that are memory mapped. The most common culpret of this type of device is a video card, which can use up to 512MB of memory for mapping.

You can tweak Vista's core to add another 4-bits of addressing capability, but you must have a 64-bit capable processor (pretty much any processor made within the past two years like Intel's Core 2 Duo or AMD's Athlon 64). Even though the math works out to address way more than 4GB (236 = 68,719,476,736), the operating system still has a cap (Windows Server can address anywhere from 8GB to 128GB depending on the flavor).

To force this new addressing method, you have to tell Vista to boot using this new parameter. Vista no longer uses a BOOT.INI file as previous versions of Windows did, so you must modify the boot file using a built-in Vista tool called BCDedit: 1)Open a command prompt (Press Window key + R to open the Run dialog, and then type cmd). 2) Press Ctrl + Shift + Enter to execute (this allows you to run cmd in administrative mode if you haven't already setup up a permanent admin mode). 3) Type BCDedit /set PAE forceenable.

This PAE flag (Physical Address Extension) tells the Vista core to use an additional 4 bits of addressing, which in theory allows the OS to see all of the RAM you have available. We're not out of the woods yet, because once you reboot you may find that Vista still doesn't see all of your RAM. You can turn PAE off again by typing BCDedit /set PAE forcedisable or BCDedit /set PAE default.

In related news, Bit-Tech published a feature titled 64-bit: More than just the RAM, which covers the pros and cons of an x86-64 computing environment.