4GB Ram

You can install as much RAM as you want... however, if you want to be able to actually make use of it, you'll need a 64-bit OS for 4GB plus. Also, if you plan on going 64-bit, I recommend Vista over XP.

No. By definition, 32-bit OSes are limited to 4GB of total address space. With all the latest patches, both XP and Vista 32-bit will display 4GB if you have 4GB installed... but this is for display only... they won't actually be able to use all 4GB.

The answer is "It Depends".

What's happening is there is a limit of address space that physical resources are bumping into.

A 32 bit operating system (XP, Vista, whatever) has 4GB worth of addresses to use, and MORE THAN JUST RAM that needs it.

Figure 500~750MB worth of addresses are needed for general system usage - Bios, communications, PCi resources, etc.. And since video memory is also memory mapped, add the needed space for your video cards - 512MB, or whatever. (Yes, I know Crossfire/SLI have a shared buffer that isn't memory mapped - thanks). So let's say that you're using half a gig worth of addresses for system use, and another half a gig for video. So this very basic system is already using 1GB worth of address space, and we haven't even touched any RAM yet... And we're already in a situation where the max memory it could possibly recognize is about 3.0GB.


Now - 3GB is still a 50% increase from 2GB, and will certainly make a difference since Windows can dedicate a full 2GB of RAM to an application and still have memory for itself. You can get there a couple ways - The first is to use a pair of 1GB sticks and a pair of 512 MB sticks, for a total of 3GB. But it's sometimes hard to match voltages, timings, and IC types, and that can potentially lead to instability issues. Then you consider that a pair of 2GB sticks isn't much more money. So it may be easiest to accept that, although you don't quite use it all, it's still a lot better than just 2.

At the level of address usage in my example and perhaps a little more, it's probably worthwhile to just accept that the OS can't address the full 4GB. But if you plan on a honkin' big single GPU, or an SLI/Crossfire setup then you're going to have to make allowances for that. The bigger you go here will directly impact how much memory space is left. So either you're going to have to use less and less RAM, or you will have to use a 64 bit OS in order to have the necessary address space to communicate to all the installed devices.