The 4GB limit refers to total addressable memory space and not just the RAM installed. It’s a total made up of system RAM, graphics RAM, PCI memory range, ACPI and a few other bits and pieces. And if you think that your system RAM is the only significant factor, think again. You can buy graphics cards fitted with 1GB of RAM, and if you wanted (and had the spare cash!) you could fit three of these into a system. There’s not a 1 to 1 relationship between graphics memeory consuming system RAM, but the larger the memory, the more the card sets aside for itself. Now that 4GB limit doesn’t leave you a lot of space for system RAM. For more information, check out an excellent post by tech guru Mark Russinovich. Here’s a small extract from Mark’s blog:
The consumption of memory addresses below 4GB can be drastic on high-end gaming systems with large video cards. For example, I purchased one from a boutique gaming rig company that came with 4GB of RAM and two 1GB video cards. I hadn’t specified the OS version and assumed that they’d put 64-bit Vista on it, but it came with the 32-bit version and as a result only 2.2GB of the memory was accessible by Windows. You can see a giant memory hole from 8FEF0000 to FFFFFFFF in this Meminfo output from the system after I installed 64-bit Windows.