When upgrading servers, you may encounter this question : Should we stay at at 32-bit OS, or 'upgrade' to a 64-bit OS?
To me, the big advantage of a 64-bit OS is the ability to address (use) more than 4 gigabytes of RAM. I sometimes bump into that limit on my desktop,
so I think it's best to have a 64 bit OS for servers.
Larger memory space isn't the only advantage, however. From Wikipedia :
A common misconception is that 64-bit architectures are no better than 32-bit architectures unless the computer has more than 4 GB of main memory. This is not entirely true...
In my opinion, any 'real' database server doing real work must be on a 64 bit system without question. This allows the DB to address more space and run more efficiently. But in many cases, we just need to shuttle info back and forth to a large database, then you can get by with a 32-bit database client application.
But if you need to run many instances of various processes, this may require a lot of memory, and having a base OS of 64 bits will allow you to grow.
I don't know how a 64-bit OS handles 32-bit apps. Maybe anything 32-bit must be run in the lower 4 GB and only true 64-bit apps can use higher memory. It's quite possible one may not be able to really use more than 4GB RAM on until you recompile/rebuild everything 64 bit.
So if you have a lot of compiled 32-bit applications and don't want to recompile to 64 bits, I think if you can get your mostly 32-bit stuff (with 32-bit client/RPMs) running on a 64-bit OS it'd be a good step forward towards eventually getting everything on 64-bit, which you may need later.