This question has come up in the last hour or so with one of my customers and I thought it was worth mentioning as a reminder.
The recommend page file size for an Exchange server is 1.5x memory up to a maximum size of 4095Mb. For example, if you have 1Gb RAM, we would recommend a page file size of 1.5Gb.
If you have the maximum RAM that Exchange can utilize (4Gb), the page file size should be 6Gb. As we have a limitation on the page file size, you will need to create a second page file on a different volume.
From a performance perspective, it is recommend to have each page file on a seperate physical volume.
For more information, these two articles are particularly interesting:
Configuring page files for optimization and recovery in Windows Server 2003, in Windows 2000, and in Windows NT
Page file size is too small