Vista 64 will use all the system RAM ALL the time. Any RAM you add is accounted or 'cached' for service or use by the system. Vista uses Superfetch unlike XP's prefetch which does none of this. Sorry, I really don't know how to answer you question in a more technical manner.
SuperFetch does more than caching. Windows Vistavista runs a SuperFetch service that analyzes your application behavior and usage patterns, meaning that it tracks which applications you request the most. A good example would be your activity as you start the PC in the morning: You launch Outlook to fetch email, a messenger, a web browser and probably additional applications such as a development environmenthttp://en.wikipedia.org/wiki/Integrated_development_env... . If you do this repeatedly and ideally in the same order, SuperFetch will recognize this and then proactively populate these applications into all available main memory the next time you start the PC. You should only wait for a few minutes before you commence work to give the SuperFetch service the time to "superfetch" your applications.
See Readyboost in the same article. Ready boost allows the system RAM even more cache space at faster speeds than a lesser OS would be accessing the HD for what Readyboost may already have cached in a quick flash drive.