Configuring memory for SQL Server 2005 Instances - 64 bit

This may sound like a very elementary question but I am kind of baffled so here goes..

I am running 4 Instances of SQL Server on one box.   The Box is Windows 2008R2 64 bit with 24 GB of memory.  There is one Default Instance of SQL Server 2005 x64, One of the named Instances is SQL Server 2005 x32 and the other two Instances are SQL Server 2005 x64.  

I have each configured to have a max memory setting of 2048mb.  

when I check the processes at the Server level, one of the sqlserver.exe processes is using 17GB of memory.    

Obviously I have something configured wrong. Maybe I didn't enable AWE for one of the Instances??    

How do I determine which Instance is the culorit and what do I do about it?