We have 64-bit Windows server and we need to explicitly enable the 32-bit mode in the Application Pool used by our web site, because the MS Jet Engine Data Provider used to import data from Excel does not run.
I am not sure, if we enable the 32-bit mode will we still get the advantage of 64-bit Server?
In IIS 7.x, there is a Advanced Settings property of the application pool called "Enable 32-bit Applications". If you run the application as a 32-bit applications it should be able to use 4 GB but obviously no more. So, in that respect, it can take advantage of running on a 64-bit server. On a 32-bit server (without special switches) it would only be able to use the 2GB of address space allocated to applications (the other 2 GB is presumed to be used by the OS).
ACE driver 64 bit is now available as a replacement of Jet driver. So you can use it instead,
http://blogs.msdn.com/b/psssql/archive/2010/01/21/how-to-get-a-x64-version-of-jet.aspx
(Read the last few paragraphs.)
来源：https://stackoverflow.com/questions/5216192/enable-application-pool-32-bit-mode-on-64-bit-server