When your Exchange Server computer has other functions to perform, it is adviseable to limit the amount of memory that Exchange Server uses.
By default, Exchange Server has access to all the available RAM, which significantly impacts the performance of other applications.
To limit the amount of RAM that Exchange Server uses, run the Microsoft Exchange Performance Optimizer. Set the amount of memory you want Exchange Server to use, but not less than 32MB.
I know of no way to manually set the registry values that the opimizer calculates.