As I wrote previously in my blog about the Windows Vista Ready Boost feature in Windows Vista, its memory model is vastly different to previous versions of Windows and indeed Windows XP. Essentially it splits the total physical RAM in a machine in roughly half, one half for programs and one half for a "cache" of what it loads from the hard drive. Needless to say this results in less overall Ram for applications to use and as a consequence requires a PC to have more physical memory in the system to give a comparable experience to what the user would be used to on, say, Windows XP on the same machine.
Is this the real reason why this memory model was adopted? Vista will drive users to 4GB RAM, and the 4GB limit will driver users to 64-bit