if you place the page file on a separate partition you don't have to worry about fragmentation.
if you let windows manage the swap size or set a minimum different than the maximum, you might encounter brief periods of unresponsiveness as windows extends/truncates the swap file. this may be undesirable on a server. although windows is usually pretty good at managing it and I've only personally experienced 2 such instances in a very long time.
the drive on which the NTDS database is stored will have 'write cache' disabled. writes to that drive will be 50-70% slower. do not store the swap file on the same drive.
the general rule is 1.5 times your ram, up to twice the amount for some application but not more. you can set at 1.5 and monitor swap file usage in your environment and tweak.
Exchange servers have an exception here.
|Paging file size||The page file size minimum and maximum must be set to physical RAM plus 10 MB||The recommended page file size also accounts for the memory that's needed to collect information if the operating system stops unexpectedly. On 64-bit operating systems, memory can be written as a dump file to the paging file. This file must reside on the boot volume of the server.|
For more information about the configuration options that are available for memory dump data, see Knowledge Base article 254649, Overview of memory dump file options for Windows Vista, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows XP, and Windows 2000.
Exchange 2010 System Requirements