Note (6apr2011): Setting the MsExchESEParamCacheSizeMax only doesn’t produce the required result as of Exchange 2010 SP1. For more information on how to limit the database cache size in Exchange 2010 SP1, see Limiting Exchange 2010 SP1 Database Cache.
I received a question from someone implementing Exchange 2010 who was surprised to see Exchange taking up all available memory. This is because in Exchange 2010 (2007 as well) memory allocation is dynamic, contrary to Exchange 2003 and earlier versions where, depending on the situation, you had to fiddle around with boot.ini switches like /3GB to make memory available to Exchange. Also, the maximum database cache size was limited in Exchange 2003 to around 1.2 GB due to virtual address space limitations (see MSKB 815372).
The main reason Exchange 2007/2010 claims memory for its database cache is performance. The more memory is assigned to the database cache, the less I/O’s are generated because things can be dealt with in-memory and the database cache becomes more effective. When a certain amount of transactions has been reached, changes will be physically written to databases (so far they’ve been stored in-memory and written to transaction logs). This limit is called the log checkpoint depth target.