Exchange will use all the memory it can find, so if you're worried that its using most of your server ram then don't be - it's working as intended. If it's dipping heavily into the pagefile then your best bet is to buy more RAM for the server or reduce its workload. Altering the pagefile settings aren't likely to help you much - the database takes a big performance hit from having to go out to disk too often.
If you add more RAM then exchange will use all of that too, if it has a need to. It's essentially a database server, and like most database servers it will use as much RAM as it can lay its hands on because the more of the database that is cached in RAM, the faster it will perform. This is a good thing if the system is a dedicated exchange server.
If you have a need to limit the RAM used then this is possible too, but I suggest you avoid this if possible:
To limit the amount of memory that Exchange uses, do the following (source):
Right click on ADSI Edit and click Connect to…
Under Select a well know Naming Context, select Configuration
Navigate to Configuration\Services\Microsoft Exchange\\Administrative Groups\<”Exchange
Set it it to the number of pages to maximize the Database Cache to. Note that Exchange 2007 works with 8 KB pages and Exchange 2010 with 32 KB pages!
Restart the Microsoft Exchange Information Store service for the
change to become effective.
NOTE: For instance, if you want to limit the Database Cache to 4 GB of
an Exchange 2010 server, set msExchESEparamCacheSizeMax to 131072 (4
GB = 4.194.304 KB / 32 KB). That’s equal to 1,048,576 for each
gigabyte of memory. 1,048,576 * 4 = 4,194,304.
To configure 5GB of memory, (1,048,576 * 5 = 5,242,880 / 32 = 163840)
. msExchESEparamCacheSizeMax to 163840