Current env:
RHEL 4; x86 (32-bit)
DB2 v9.1 32-bit


To be upgraded to:
RHEL 6; stay on x86 or upgrade to x86_64 (64-bit)
DB2 v9.7 32-bit or 64-bit depending on the OS


The application will remain as 32-bit.



Does it make sense to upgrade OS to 64-bit (and v9.7 instance will become 64-bit) if the database doesn't require more memory than what it's currently using (based on 32-bit address space limitations)?

As far as I know, 32-bit applications can work with 32-bit or 64-bit instances:
IBM DB2 9.7 for Linux, UNIX and Windows Information Center

Do you see any problem with 32-bit appl and 64-bit instance?

No problem with 32-bit client accessing a 64-bit DB2 server.

I personally would not want to deal with a 32-bit DB2 instance because the (approximately) 1.2 GB memory limitation under Linux is too constricting for high performance applications/database. I want to have 100% bufferpool hit ratio 99.9% of the time. I have some databases with 64 GB bufferpool memory.