> For all I know,
>
> - The client application must be same bit, as Firebird client.
> Application 32 bit -> Firebird client 32 bit
> Application 64 bit -> Firebird client 64 bit.

Yes.

> - Any client (32/64 bit) can connect to any Firebird server (32/64 bit).
>
> - 32 bit OS -> 32 bit application
> - 64 bit OS -> 32 bit/64 bit application.

Yes. Bitness between the Firebird client and Firebird server is
completely transparent.

> The question: on 64 bit Windows (more than 4G mem), with 32 bit
> application and Firebird client, gainful the Firebird 64 bit server usage ?

If you are forced to run Firebird Server 32-bit, e.g. due to UDF
dependencies, on an 64-bit OS, then go with the one process per
connection model aka Classic Server.

The bitness of the Firebird client doesn't matter. Your client
application will easily connect to a Firebird 32-bit and/or 64-bit server.



--
With regards,
Thomas Steinmaurer
http://www.upscene.com/

Professional Tools and Services for Firebird
FB TraceManager, IB LogManager, Database Health Check, Tuning etc.