Itanium is tru64 bit processor, so you should never run a 32 bit application on Itanium, it does not emulate a 32 bit environment like a Xeon 64bit processor does.
When we talked with HP they warned us about this limit of 64 bit application. We then had to verify that ALL applications, virus, backup, hba software, custum apps, we written and supported in a 64 bit environment. After we cleared all the software our system came preloaded with 2003 Data Center 64bit
Therefore I am not 100% sure you could even load 32bit OS on the server. The one 32 bit app that got passed us (by EMC) crashed the box multiple times.
Also, I believe, exchange 2007 requires OS to be 64 bit as well. i might be wrong, we just moved away from Exchange over to Domino.
PER MS for Exchange2007
Operating system on a computer that has a 32-bit processor for testing and training environments only (This is not supported in production environments.)