Windows always tries to allocate memory based on physical memory(RAM)+ virtual memory (HDD "swap file") + software priority. However, where XP, and even Vista 32bit, limit the amounts of RAM available to various programs (due to the 32 bit addressing issue), Vista 64 doesn't have this problem. There, Vista 64 tries to maximise physical memory usage, which is much faster than virtual memory.
For instance, if your listening to an MP3 and surfing the 'net, and you have an anti-virus software running in the background, the OS "sees" that your not using much memory. Therefore, it will allow all the other programs to use more physical memory and reduce the amount of "swapping" to and from the HDD. However, once you fire up that 3D shooter, it will "shrink" all the background tasks, which is basically what the "swap file" is for, and makes room for all the game files to be loaded into memory. Once you quit the game, the OS allows things to re-expand into memory as needed.
Does this help?