Unless I am reading you wrong, all of your memory is installed physically but you are only using 800 MB of th total available memory. The rest of it is cached ready to be used if need be. Whatever you were using previous to the memory upgrade will not change as a result of the memory upgrade.
The amount of phsical memory used is equal to what is running in the system at the present time. So if you open an application that is not currently running you will see an increase in the amount of physical memory being used. Vista remembers what applications you use most frequently ard caches those applications into memory at startup to reduce the time necessary to load the application when you need it. This is why Vista usually takes so long to boot up.
If you want to know your total phsical memory installed and what has been cached, and therefore by deduction what is being used look in task manager:
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