Each 32-bit process is limited to a 4gb address space. This is usually split in half, where 2gb is the amount of actual memory the process can allocate. It is also possible to set the application to be able to use 3gb of memory but that has both benefits and drawbacks.
Each zone a server is running is a different process, so it is possible for multiple 32-bit zone processes on a 64-bit OS to collectively use as much memory is available. It's pretty unlikely you'd have a single zone process that needed more than 2gb of memory unless there was a giant memory leak.
Anyone running 64-bit on a highly populated server may have a different opinion of the performance versus 32-bit, but for most I'd say they are pretty much the same.
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