Should I install 32-bit database or 64-bit database?

(I've read a lot about 64-bit versus 32-bit OS/Apps, but this question is specifically in regards to databases.)

I'm trying to understand the pros and cons of 32-bit versus 64-bit databases, and namely, under what conditions that it starts to make sense to use 64-bit installations.

The database systems that I am interested in are: SQL Server 2008, MySQL, and PostgreSQL 9.0.

I have read that pre-9.0 versions of 开发者_JAVA百科PostgreSQL only comes in 32-bit for Windows, and this article about running 32-bit PostgreSQL on 64-bit Windows clears up some of my confusion, but I'm looking for more info.

When would I benefit from using 64-bit databases (i.e. database size/disk space, available system memory, types of data sernarios that are known to benefit from it, which database engine being used, etc.)?

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Assuming you have a 64-bit capable OS and processor, the most important advantage of 64-bits is the ability to use more than 4GB of memory.