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The problem is that 32-bit Windows can address 4GB of memory but that includes all of the system memory and devices in your PC. Again, the specifics are rather complicated but essentially many parts of your computer have small bits of memory that need to be addressed so even in a basic system you'll never see the full 4GB. Factor in a high-end graphics card and things actually get worse as the 512MB, 76MB or even 1024Mb of memory on the graphics card has to be included in the addressable space. This means a high-end system with 4GB of RAM may only be able to use 3GB of RAM.
This is perfectly illustrated by our screen grab from 32-bit Vista. In fact we had a 4GB Corsair kit installed but Windows was only ever going to show 3.5GB on this set-up and there was 512MB of memory to be deducted for the graphics card, hence the 3GB figure.
So what would happen if we installed a pair of Radeon HD 3870 graphics cards in CrossFire on our Abit IP35 Pro? That's another 512MB of memory to account for which would reduce the total to 2.5GB so a 4GB kit would only show a small increase over the 2GB that most of us use at present. If you're an avid gamer who is planning on going the whole hog by installing four graphics cards in CrossFire X you can expect to have 2GB of video memory which would only leave 1.5GB of address space for system memory.
And that's not the worst of it!
On the one hand the amount of system memory that is available is going to diminish as you increase the graphics power in your PC yet on the other you can be sure that games and applications will demand more from your system in the future, including the memory.
If you run software such as Process Explorer, from Sysinternals, in the background you'll see that games such as Crysis can use in excess of 1GB of memory while screen shots that we've seen of Company of Heroes show it has been allocated a colossal 3GB of RAM. That's a hefty chunk of the available resources and if you hit an especially heavy-duty map - or the developers make a mistake - it's easy to see that you can run out of memory.
Fundamentally, you're unlikely to see a huge boost in performance moving from 2GB to 4GB if you're running a 32-bit operating system. However, that's not to say there will be no benefit.
As mentioned, if you take a reasonably high-end system with a graphics card with 512MB of onboard memory, you're likely to end up with about 3GB of useable system memory, which is still an improvement over 2GB. Moreover, if and when you you do move to a 64-bit operating system (64-bit operating systems are able to properly use up to 17,179,869,184GB of RAM) you'll have plenty of RAM to go along with. So, if you're still using Windows XP, moving straight to 64-bit Vista in a few months (when service pack 1 is released, perhaps) is the perfect upgrade path.