Interesting post by Hilton Locke about why installing 4 GB of RAM on a 32-bit box doesn't actually give you 4 GB of RAM. Instead it gives you something like 3 GB of RAM. A 32-bit operating system should be able to access 4GB. Worse, a 64-bit operating system might not be any better. What's happening? The short answer is that a lot of space it taken up by memory-mapped IO. The RAM can't be seen because that space is used for IO devices. I hadn't considered that. For the long answer, read the whole post. There's a lot of interesting info in there.