This is another very quick common question that seems to not have any documentation.
Scenario: You go to get the latest hot fix rollup for forefront for Exchange. And you notice there are two versions of the patch. A 32 bit version and a 64 bit version. Well Exchange is 64 bit but wait Forefront is installed in the Program files(x86) directory.
So what version do you need?
Depending on your setup...
There are two patches. One of them (the x86 version) is for client machines only.
The x64 version is for the server.
The forefront product has x64 components but for the most part is 32 bit.
If you are installing on a windows vista machine and it has the client installed you would grab the x86 version.
If you are installing on the server with the actual Exchange services on it, then you install the x64 version.