You can also use the classic version of Firebird on a SMP Linux box. By
using the classic version, you can benefit from both processors. The
drawback of the classic version is that it takes more resources than the
superserver version and therefore, you will need more RAM on your server.

From our experience, one of our customers is using the following
configuration:
Dual 1000 MHz P3 with 512 MB Ram and Two SCSI 160 18 GB 15000 RPM disks.
We then Installed Redhat 7 (you can also use Suse Linux which is quite
robust from what I read).
The network is 100 Mb/s (I believe that if the application is designed with
the spirit of Client/Server you do not need a faster network).
This customer has 20 very active connections. The database is about 1.5 GB
and contains a lot of complex triggers and stored procedures.
So far, this configuration has proved to be extremely fast and reliable. It
has been up for quite a while without a glitch (except when they uplugged
the server while it was running for vacuming!).

I remember seing an interesting document that suggested such a configuration
and more. I believe that you can find it in the www.ibphoenix.com website. I
will try to find the exact link for that. Try visiting this site, it has
lots of excellent documents.