While reading the "Road to Snow Leopard" series on Apple Insider, I've came across a very interesting bit:
Prior to using the Santa Rosa platform, Apple sold its laptops as only supporting a maximum of 3GB RAM because of this. However, many Original Equipment Manufacturer PC assemblers represent their machines as supporting 4GB of RAM even though the operating system can't actually make any use of a big chunk of it. With hardware that only supports 32-bit addressing, no operating system can make use of the full 4GB. However, even with Santa Rosa-style hardware that can make use of the full 4GB, the mainstream 32-bit Windows Vista still won't use more than 3.2GB or less because it can't remap MMIO.
This means that 32 bit windows systems (Vista 32 bit and Xp x86) won't be able to maximize the usage of at least 25% of the RAM should the user choose to ugrade based on recommendations by "experts" wanting to upsell more stuffs. The issue is made worse by computer systems intergrators trying to make more profit from unknowing users as stated in the following paragraphs:
One developer we consulted about the issue noted, "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."
"Microsoft even changed the way the OS reports the amount of RAM available; rumor is, due to pressure from OEMs," the developer told us. "In Vista and prior, it reported usable RAM, while in SP1 they changed it to report installed RAM ignoring the fact that much of the RAM was unusable due to overlap with video memory." And so many PC users are installing 4GB of RAM in their PCs and thinking that it is being used by the system, when in fact it is no more beneficial than if the RAM were simply poked halfway into the CD slot.
For example, Dell's top of the line $5799 Alienware gamer PC comes standard with a 1GB video card, 2GB of RAM, and 32-bit Windows Home Premium. That means the system can only possibly use 2.3GB of RAM, but Dell "recommends" users spend $250 (or $8 per month with financing) to buy a 4GB upgrade (below) that will offer them little more than bragging rights, as the 1GB video card and the roughly 750MB of other MMIO will make the extra 2GB unusable. Dell says "Upgrading your memory allows you to take full advantage of system capabilities as well as increasing system efficiency," but that's simply not true on this page.
Simple solution here. If you want to install more than 3GB of RAM, get a 64 bit Windows, else your money invested is wasted.
Do note that many older devices won't work with Windows Vista x64 because of non-existing drivers. Notable are the webcams from Logitech (they are trying to sell me newer webcams instead of writing drivers that supports vista :S).
ps: On the brighter note, I am pretty happy with my new Macbook. It certainly is a lot user-friendlier than a comparable windows system, to the point that I kinda miss the gesture based UI when i get back to my old system for maintenance