IanG on Tap: 3GB or 4GB - a 'Virtual' Memory Upgrade:

Presumably the reason PC architecture machines didn't do this is that this mapping layer increases costs, and makes it harder to achieve performance goals. So for years, the address map of the PCI bus and the address map of the system bus have been one and the same thing in the PC. ... For a long time this wasn't a problem, because there was a whole 4GB of address space, so devices typically lurk up in the top 1GB of physical address space, leaving the bottom 3GB for memory.

And 3GB should be enough for anyone right?

So what actually happens if you go out and buy 4GB of memory for your PC? Well, it's just like the DOS days - there's a hole in your memory map for the IO. ... So the bottom 3GB of your memory will be available, but there's an issue with that last 1GB.