Extra extra: the reason 32-bit XP-SP2|Vista limit address space to 4GB (that is, PCI space combined with memory) is because they don't trust driver writers to do the right thing when DMA buffers straddle 4G boundary.

Who modded parent up? Totally misleading.

Windows /3GB switch controls amount of virtual memory available to a process. Not physical. Virtual. The rest of virtual memory space goes to OS. Low memory (2 or 3 GB) is user process visible (ring3) memory. The upper 2 or 1 GB are ring0 memory. No need to change memory mapping (no need to flush TLB) on a simple syscall that goes back to the same process. Linux does the same thing. [OSX prefers to give userland 4GB virtual memory]

The fact you need /3GB switch illustrates that there are applications that need more than 2GB of virtual space per process. What if you run a number of these apps? Does 4GB look like a lot of memory now?

In fact, Crysis on 64bit Vista with very high settings and res can start swapping to disk because it outgrows 4GB. This is a single app plus graphics drivers.

And, the #1 reason to have virtual memory: memory-mapped files.