Much ado about the /3GB switch
Do you need to put the /3GB switch on to your Exchagne server? Most definitely if you have more than 1GB of physical memory. Don't forget the /USERVA=3030 switch, too. This works on all Windows 2003 servers (well, Server, Enterprise Server, and Datacenter). The /3GB switch works on all Windows 2000 Advanced Server and Datacenter servers, but not standard edition.
Here is an example of one of mine.
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows Server 2003, Enterprise" /fastdetect /3GB /USERVA=3030
Evan Dodds explains this really well in his blog.
The Exchange team also has some indepth information and links on this courtesy of Nino Bilic