Who would have ever thought we’d run into the 32-bit limitation. The long and short is that with all versions of Windows, 750MB at the top of memory is useless because it is used for I/O and if you have a 1GB video card, that is memory mapped, so that means you can use at most 2.25GB of real memory in most high end machines on Windows. Sigh. Now that is really a ripoff.
This is why the highest end Mac only has 3GB of real memory, you can’t use more with today’s Leopard, although the newest Santa Rosa chipset 2H2007 machines from Apple use PAE to get around this problem. Windows XP and even Vista don’t support it. Another reason to buy a Mac.
AppleInsider | Road to Mac OS X Snow Leopard: 64-bits, Santa Rosa, and more“consumers are being scammed by [PC] OEMs on a large scale. OEMs will encourage customers to upgrade a 2GB machine to 4GB, even though the usable RAM might be limited to 2.3GB. This is especially a problem on high-end gaming machines that have huge graphics cards as well as lots of RAM.“