yes, if you have one application that needs more than 4G itself (not counting memory used by the kernel internally, or used by the kernel to buffer disk I/O), then you need to use AMD64, not x32
a large database is a perfect example of a situation where you would want the full 64 bits available.
given these other memory usesin a system, it's very likely that a machine with 6-8G of ram that's dedicated for database use could still be very happy with x32
however, if you are splitting the database up using sharding (where you have multiple database instances, which could live on separate machines, including virtual machines), it's very possible that each one will only need 4G or less of address space even with far more ram.
also, if you have a database like postgres that used multiple processes (instead of multiple threads), you should recognize that each process can have 4G of address space, so unless you have a huge amount of shared memory allocated, 4G per process may be a very comfortable limit.