> What the best firebird version for computer server IBM system 3400 + win server 2003 ?

Talking about Firebird 2.5:

32-bit OS => Classic
64-bit OS => Classic or SuperClassic.

Both, Classic and SuperClassic support SMP environments. According to
some tests from one of Dmitry's presentation, SuperClassic seems to be
(a bit) faster in a TPC benchmark. Also look here:
http://www.slideshare.net/mindthebird/firebirdperformancetesting

Classic and SuperClassic differ in the used connection/process model and
both offer different things in respect to administration. E.g. if a
connection crashes in Classic, it won't affect other connections. On the
other side, in SuperClassic, you can safely shutdown all connections.

Have a look at my architecture comparison sheet:
http://blog.upscene.com/thomas/index.php?entry=entry091016-133211


Good luck.


--
Best Regards,
Thomas Steinmaurer
LogManager Series - Logging/Auditing Suites supporting
InterBase, Firebird, Advantage Database, MS SQL Server and
NexusDB V2
Upscene Productions
http://www.upscene.com
My blog:
http://blog.upscene.com/thomas/