Memory usage myths

Physical vs virtual
Regardless of how much actual [i]physical[/i] RAM you have installed, applications run in their own virtual address space. For a 32-bit application under Windows, this is 2GB. The application doesn't have to use that much, but can if it wants to and it's up to the operating system to deliver while also keeping all other applications (including system processes) happy.

And you know what happens when the combined load of memory requested by applications exceeds physical RAM: the operating system starts swapping out to disk (actually the science of memory management will often pre-emptively swap out background programs to allow foreground ones to be more responsive as memory is required, and other such tricks).

If, of course, you do have plenty of RAM installed and the application does want to allocate lots of it, then it can do so, but it's limited by the addressable space the application can see. By default Windows limits a 32-bit application to 2GB, and if it tries to allocate more than this it will fail. Sometimes gracefully, sometimes spectacularly. Either way, it doesn't matter if you have more physical RAM, it can't be used by the program.

And this is true even if you're running 64-bit Windows. Remember, with 64-bit Windows the operating system itself can see and use more than the 4GB limitation of 32-bit Windows, but that doesn't mean applications can. A 64-bit application has a very large addressable space assigned to it by Windows - a gigantic 8TB. But as mentioned before, even today the majority of applications for Windows are still 32-bit, and with very rare exceptions, so too are games. So having 6GB to play COD4 is moot.

Overall, the breakdown looks like this for addressable space per program:

32-bit application on 32-bit Windows: 2GB

32-bit application on 64-bit Windows: 2GB

64-bit application on 64-bit Windows: 8TB

Which isn't good news for modern games. More and more are becoming demanding of available memory, but without the move to 64-bit - which so few game developers are willing to do - they will either not benefit from more memory (even using it as a local cache) or they will simply bomb out when they reach the 2GB limit of their addressable memory. Which is precisely what happens with Flight Simulator, STALKER, and Supreme Commander, and why some enterprising users have taken it into their own hands.